Magnetic recording medium

ABSTRACT

An object is to provide a magnetic recording medium having excellent traveling stability and a thin total thickness.The present technology provides a tape-shaped magnetic recording medium including: a magnetic layer; an underlayer; a base layer; and a back layer, in which the number of recesses having a depth of 20% or more of the thickness of the magnetic layer is 10 or more and 200 or less per surface area of 1600 μm2, the base layer includes a polyester as a main component, a surface on a side of the back layer has a kurtosis of 2.0 or more and 4.4 or less, the magnetic recording medium has an average thickness tT of 5.6 μm or less, the magnetic recording medium includes a lubricant, the magnetic recording medium has pores, and the pores have an average diameter of 6 nm or more and 11 nm or less when the diameters of the pores are measured in a state where the lubricant has been removed from the magnetic recording medium and the magnetic recording medium has been dried.

TECHNICAL FIELD

The present technology relates to a magnetic recording medium.

BACKGROUND ART

For example, with development of IoT, big data, and artificial intelligence, the amount of data collected and stored has increased significantly. A magnetic recording medium is often used as a medium for recording a large amount of data.

Various techniques relating to a magnetic recording medium have been proposed so far. For example, Patent Document 1 below discloses a technique relating to a magnetic recording medium having a magnetic layer containing at least a binder and magnetic powder on at least one main surface of a nonmagnetic support. In the magnetic recording medium, the thickness of the magnetic layer is 0.12 μm or less, the root mean square surface roughness (Rq) of the surface of the magnetic layer formation surface is 4.0 nm or less, and skewness (Sk) in the surface profile of the magnetic layer formation surface is −1 or more and +1 or less.

CITATION LIST Patent Document Patent Document 1: Japanese Patent Application Laid-Open No. 2006-65953 SUMMARY OF THE INVENTION Problems to be Solved by the Invention

A magnetic recording medium is housed in, for example, a magnetic recording cartridge. In order to further increase the recording capacity per magnetic recording cartridge, it is considered to make a magnetic recording medium (for example, magnetic recording tape) housed in the magnetic recording cartridge thinner (to reduce the total thickness), and to increase the tape length per magnetic recording cartridge.

However, a magnetic recording medium having a thin total thickness may have a problem in the smoothness of a magnetic layer side surface thereof. For example, when the magnetic layer side surface is smooth, a frictional force between the magnetic layer side surface and a magnetic head may increase. In particular, the frictional force tends to increase in a case where repeated recording or reproduction is performed. Furthermore, a magnetic recording medium having a smooth magnetic layer side surface may easily cause winding deviation.

Therefore, a main object of the present technology is to control a frictional force and to prevent winding deviation in a magnetic recording medium having a thin total thickness.

Solutions to Problems

The present technology provides a tape-shaped magnetic recording medium including:

a magnetic layer; an underlayer; a base layer; and a back layer, in which

the number of recesses having a depth of 20% or more of the thickness of the magnetic layer is 15 or more and 200 or less per surface area of 1600 μm²,

the base layer includes a polyester as a main component,

a surface on a side of the back layer has a kurtosis of 2.0 or more and 4.4 or less,

the magnetic recording medium has an average thickness t_(T) of 5.6 μm or less,

the magnetic recording medium includes a lubricant, and

the magnetic recording medium has pores, and the pores have an average diameter of 6 nm or more and 11 nm or less when the diameters of the pores are measured in a state where the lubricant has been removed from the magnetic recording medium and the magnetic recording medium has been dried.

The magnetic recording medium can have a squareness ratio of 65% or more in a perpendicular direction thereof.

The magnetic layer side surface of the magnetic recording medium can have arithmetic average roughness R_(a) of 2.5 nm or less.

The magnetic layer can have an average thickness t_(m) of 80 nm or less.

The magnetic layer includes magnetic powder, and the magnetic powder can contain hexagonal ferrite, ϵ iron oxide, or Co-containing spinel ferrite.

The hexagonal ferrite can contain at least one of Ba or Sr, and the ϵ iron oxide can contain at least one of Al or Ga.

The magnetic recording medium can have a friction coefficient ratio (μ_(B)/μ_(A)) of 1.0 to 2.0, in which μ_(A) represents a coefficient of dynamic friction between a magnetic layer side surface of the magnetic recording medium and a magnetic head in a state where a tension of 0.4 N is applied to the magnetic recording medium in a longitudinal direction thereof, and μ_(B) represents a coefficient of dynamic friction between the magnetic layer side surface of the magnetic recording medium and the magnetic head in a state where a tension of 1.2 N is applied to the magnetic recording medium in the longitudinal direction.

The magnetic recording medium can have a friction coefficient ratio (μ_(C(1000))/μ_(C(5))) of 1.0 or more and 1.8 or less, in which μ_(C(5)) represents a coefficient of dynamic friction at the fifth reciprocation in a case where the magnetic recording medium in a state where a tension of 0.6 N is applied to the magnetic recording medium in a longitudinal direction thereof is reciprocatedly slid five times on a magnetic head, and μ_(C(1000)) represents a coefficient of dynamic friction at the 1000th reciprocation in a case where the magnetic recording medium is reciprocated 1000 times on the magnetic head.

The lubricant can contain a fatty acid and/or a fatty acid ester.

The fatty acid can contain a compound represented by the following general formula (1) or (2), and the fatty acid ester can contain a compound represented by the following general formula (3) or (4):

CH₃(CH₂)_(k)COOH  (1)

where, in the general formula (1), k is an integer selected from a range of 14 or more and 22 or less;

CH₃(CH₂)_(n)CH═CH(CH₂)_(m)COOH  (2)

where, in the general formula (2), the sum of n and m is an integer selected from a range of 12 or more and 20 or less;

CH₃(CH₂)_(p)COO(CH₂)_(q)CH₃  (3)

where, in the general formula (3), p is an integer selected from a range of 14 or more and 22 or less, and q is an integer selected from a range of 2 or more and 5 or less; and

CH₃(CH₂)_(r)COO—(CH₂)_(s)CH(CH₃)₂  (4)

where, in the general formula (4), r is an integer selected from a range of 14 or more and 22 or less, and s is an integer selected from a range of 1 or more and 3 or less.

The magnetic recording medium can have a coercive force H_(c) of 2000 Oe or less in a longitudinal direction thereof.

The magnetic layer can have five or more servo bands.

The ratio of the total area of the servo bands to the area of the magnetic layer side surface of the magnetic recording medium may be 4.0% or less.

The servo band can have a width of 95 μm or less.

The magnetic layer can form a plurality of data tracks, and the width of each of the data tracks can be 2.0 μm or less.

The magnetic layer may be able to record data such that a minimum value of a distance L between magnetization inversions is 48 nm or less.

The base layer may have an average thickness of 4.2 μm or less.

The magnetic layer includes magnetic powder, and the magnetic powder can have an average aspect ratio of 1 or more and 3.5 or less.

The magnetic layer includes magnetic powder, and the magnetic powder can have an average particle size of 50 nm or less.

A surface on the back layer side may have a kurtosis of 3.0 or more.

The number of recesses having a depth of 20% or more of the thickness of the magnetic layer can be 12 or more per surface area of 1600 μm².

The number of recesses having a depth of 20% or more of the thickness of the magnetic layer can be 14 or more per surface area of 1600 μm².

The pores can have an average diameter of 6 nm or more and 10 nm or less.

The pores can have an average diameter of 7 nm or more and 9 nm or less.

The present technology also provides a tape cartridge including:

the tape-shaped magnetic recording medium,

a communication unit that communicates with a recording/reproducing device;

a storage unit; and

a control unit that stores information received from the recording/reproducing device through the communication unit in the storage unit, reads the information from the storage unit according to a request from the recording/reproducing device, and transmits the information to the recording/reproducing device through the communication unit, in which

the information includes adjustment information for adjusting a tension applied to the magnetic recording medium in a longitudinal direction thereof.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view of an example of a magnetic recording medium according to the present technology.

FIG. 2 is a diagram illustrating an example of data bands and servo bands disposed in a magnetic recording medium.

FIG. 3 is a diagram illustrating an example of a servo pattern in a servo band.

FIG. 4 is a diagram illustrating an example of a servo pattern in a servo band.

FIG. 5 is a cross-sectional view illustrating a configuration of a magnetic particle.

FIG. 6 is a cross-sectional view illustrating a configuration of a magnetic particle in Modification.

FIG. 7 is a diagram for explaining a method for measuring a friction coefficient between a magnetic surface and a magnetic head.

FIG. 8 is a schematic diagram illustrating a configuration of a recording/reproducing device.

FIG. 9 is a schematic cross-sectional view of a magnetic recording medium in Modification.

FIG. 10 is an example of a TEM photograph of a magnetic layer.

FIG. 11 is an exploded perspective view illustrating an example of a configuration of a cartridge.

FIG. 12 is a block diagram illustrating an example of a configuration of a cartridge memory.

FIG. 13A is a first explanatory diagram illustrating a method for measuring the surface density of recesses in the magnetic recording medium illustrated in FIG. 1.

FIG. 13B is a second explanatory diagram illustrating a method for measuring the surface density of recesses in the magnetic recording medium illustrated in FIG. 1.

FIG. 13C is a third explanatory diagram illustrating a method for measuring the surface density of recesses in the magnetic recording medium illustrated in FIG. 1.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a preferred embodiment for carrying out the present technology will be described. Note that the embodiments described below exemplify representative embodiments of the present technology, and the scope of the present technology is not limited only to the embodiments.

The present technology will be described in the following order.

1. Description of the present technology

2. Embodiment of the present technology (example of application type magnetic recording medium)

(1) Configuration of magnetic recording medium

(2) Description of each layer

(3) Physical properties and structure

(4) Method for manufacturing magnetic recording medium

(5) Recording/reproducing device

(6) Cartridge

(7) Effect

(8) Modification

3. Examples

1. Description of the Present Technology

The present inventors have studied various magnetic recording media each having a thin total thickness. As a result, the present inventors have found that a magnetic recording medium having a specific configuration can suppress an increase in a coefficient of dynamic friction due to repeated recording or reproduction and can prevent occurrence of winding deviation. That is, the magnetic recording medium according to the present technology includes: a magnetic layer; an underlayer; a base layer; and a back layer, in which the number of recesses having a depth of 20% or more of the thickness of the magnetic layer is 10 or more and 200 or less per surface area of 1600 μm², the base layer includes a polyester as a main component, a surface on the back layer side has a kurtosis of 2.0 or more and 4.4 or less, the magnetic recording medium has an average thickness t_(T) of 5.6 μm or less, the magnetic recording medium contains a lubricant, the magnetic recording medium has pores, and the pores have an average diameter of 6 nm or more and 11 nm or less when the diameters of the pores are measured in a state where the lubricant has been removed from the magnetic recording medium and the magnetic recording medium has been dried.

Since the number of the recesses is within the numerical range, the kurtosis is within the numerical range, the magnetic recording medium includes a lubricant, and the average diameter of the pores of the magnetic recording medium is within the numerical range, an increase in the coefficient of dynamic friction of the magnetic layer side surface due to repeated recording or reproduction can be suppressed, and winding deviation can be further prevented.

The number of recesses having a depth of 20% or more of the thickness of the magnetic layer is 10 or more and 200 or less per surface area of 1600 μm² of the magnetic layer side surface of the magnetic recording medium. The number of the recesses may be preferably 12 or more, more preferably 14 or more, and still more preferably 15 or more.

The number of the recesses may be preferably 180 or less, more preferably 160 or less, and still more preferably 140 or less or 120 or less. The number of the recesses may be, for example, 12 or more and 160 or less, and more preferably 14 or more and 120 or less.

The number of the recesses within the above numerical range contributes to suppressing an increase in the coefficient of dynamic friction. For example, the number of the recesses equal to or more than the lower limit value described above can suppress an increase in the coefficient of dynamic friction of the magnetic layer side surface due to repeated recording or reproduction.

Furthermore, the number of the recesses within the numerical range described above also contributes to improvement in electromagnetic conversion characteristics of the magnetic recording medium. For example, SNR can be improved by setting the number of the recesses to be equal to or less than the upper limit value described above.

The magnetic recording medium according to the present technology has pores, and the pores have an average diameter of 6 nm or more and 11 nm or less when the diameters of the pores are measured in a state where the lubricant has been removed from the magnetic recording medium and the magnetic recording medium has been dried. The average diameter is preferably 10 nm or less, and more preferably 9 nm or less. The average diameter is preferably 6.5 nm or more, more preferably 7 nm or more, still more preferably 7.5 nm or more, and particularly preferably 8 nm or more. The average diameter is more preferably 6 nm or more and 10 nm or less, still more preferably 6.5 nm or more and 10 nm or less, and further still more preferably 7 nm or more and 9 nm or less.

The average pore diameter within the numerical range described above can suppress an increase in the coefficient of dynamic friction after repeated recording or reproduction is performed. In a case where the average pore diameter is outside the numerical range described above, friction between the magnetic recording medium and the head gradually increases as the magnetic recording medium travels, and traveling stability may deteriorate. It is considered that an appropriate amount of lubricant appears on the magnetic layer side surface due to the average diameter within the numerical range described above, and this contributes to improvement in traveling stability of the magnetic recording medium having a thin total thickness. The pores may be formed, for example, on a surface of the magnetic recording medium, more particularly on a surface of the magnetic layer side. The pores may be present, for example, in the magnetic layer. The pores present in the magnetic layer may be formed only in the magnetic layer, or the pores formed in the magnetic layer may extend to another layer, for example, to the underlayer.

Furthermore, the average pore diameter within the numerical range described above can suppress a change in the coefficient of dynamic friction in a case where a tension applied to the medium changes during traveling of the magnetic recording medium. The magnetic recording medium of the present technology capable of suppressing the change is suitable, for example, for use in a recording/reproducing device that can keep the width of a long magnetic recording medium constant or substantially constant by adjusting a tension of the magnetic recording medium in a longitudinal direction thereof. The recording/reproducing device detects, for example, dimensions or a dimensional change of the magnetic recording medium in a width direction thereof, and adjusts the tension in the longitudinal direction on the basis of a detection result. For example, in a case where a servo track width is wider than a predetermined width, the device increases the tension in the longitudinal direction and keeps the servo track width constant, and in a case where the servo track width is narrower than the predetermined width, the device decreases the tension in the longitudinal direction and keeps the servo track width constant. In this way, the width of the magnetic recording medium is kept constant.

The kurtosis Sku of the surface on the back layer side of the magnetic recording medium according to the present technology may be 2.0 or more, preferably 2.1 or more, more preferably 2.2 or more, still more preferably 2.3 or more, and for example, 3.0 or more. Furthermore, the kurtosis Sku of the surface on the back layer side of the magnetic recording medium according to the present technology can be 4.4 or less, preferably 4.3 or less, more preferably 4.2 or less, and still more preferably 4.0 or less.

The kurtosis of the surface on the back layer side within this numerical range can prevent occurrence of winding deviation in a magnetic recording medium having a thin total thickness.

The average thickness t_(T) of the magnetic recording medium according to the present technology can be 5.6 μm or less, more preferably 5.3 μm or less, and still more preferably 5.2 μm or less, 5.0 μm or less, or 4.6 μm or less. Since the magnetic recording medium according to the present technology is such a thin medium, for example, a tape length wound around one magnetic recording cartridge can be made longer, thereby increasing the recording capacity per magnetic recording cartridge.

The width of the magnetic recording medium according to the present technology can be, for example, 5 mm to 30 mm, particularly 7 mm to 25 mm, more particularly 10 mm to 20 mm, and still more particularly 11 mm to 19 mm. The length of the tape-shaped magnetic recording medium according to the present technology can be, for example, 500 m to 1500 m. For example, the tape width according to the LTO8 standard is 12.65 mm and the length according thereto is 960 m.

The magnetic recording medium according to the present technology has a tape shape, and can be, for example, a long magnetic recording tape. The tape-shaped magnetic recording medium according to the present technology may be housed in a magnetic recording cartridge, for example. More specifically, the magnetic recording medium may be housed in the cartridge while being wound around a reel in the magnetic recording cartridge.

The magnetic recording medium according to the present technology includes a magnetic layer, an underlayer, a base layer, and a back layer. These four layers may be laminated in this order. In addition to these layers, the magnetic recording medium according to the present technology may include another layer. The other layer may be appropriately selected according to the type of the magnetic recording medium. The magnetic recording medium according to the present technology can be, for example, an application type magnetic recording medium. The application type magnetic recording medium will be described in more detail in the following column 2.

2. Embodiment of the Present Technology (Example of Application Type Magnetic Recording Medium)

(1) Configuration of Magnetic Recording Medium

First, a configuration of a magnetic recording medium 10 according to a first embodiment will be described with reference to FIG. 1. The magnetic recording medium 10 is, for example, a magnetic recording medium that has been perpendicularly orientated, and as illustrated in FIG. 1, includes a long base layer (also referred to as a substrate) 11, an underlayer (nonmagnetic layer) 12 disposed on one main surface of the base layer 11, a magnetic layer (also referred to as a recording layer) 13 disposed on the underlayer 12, and a back layer 14 disposed on the other main surface of the base layer 11. Here, out of both main surfaces of the magnetic recording medium 10, the surface on which the magnetic layer 13 is disposed is also referred to as a magnetic surface or a magnetic layer side surface, and the surface opposite to the magnetic surface (surface on which the back layer 14 is disposed) is also referred to as a back surface.

The magnetic recording medium 10 has a tape shape and travels in a longitudinal direction thereof during recording and reproduction. Furthermore, the magnetic recording medium 10 may be able to record a signal at the shortest recording wavelength of preferably 100 nm or less, more preferably 75 nm or less, still more preferably 60 nm or less, particularly preferably 50 nm or less, and can be used, for example, for a recording/reproducing device having the shortest recording wavelength within the range described above. This recording/reproducing device may include a ring type head as a recording head. The recording track width can be, for example, 2 μm or less.

(2) Description of each Layer

(Base Layer)

The base layer 11 can function as a support for the magnetic recording medium 10, and is, for example, a long nonmagnetic substrate having flexibility, and in particular, can be a nonmagnetic film. The thickness of the base layer 11 is, for example, 8 μm or less, preferably 7 μm or less, more preferably 6μm or less, still more preferably 5 μm or less, and particularly preferably 4.2 μm or less. The thickness of the base layer 11 can be, for example, 2 μm or more, preferably 2.2 μm or more, more preferably 2.5 μm or more, and still more preferably 2.6 μm or more.

The average thickness of the base layer 11 can be determined as follows. First, the magnetic recording medium 10 having a width of ½ inches is prepared and cut into a length of 250 mm to manufacture a sample. Subsequently, layers of the sample other than the base layer 11 (that is, the underlayer 12, the magnetic layer 13, and the back layer 14) are removed with a solvent such as methyl ethyl ketone (MEK) or dilute hydrochloric acid. Next, the thickness of the sample (base layer 11) is measured at five or more points using a laser hologage (LGH-110C) manufactured by Mitutoyo Corporation as a measuring device, and the measured values are simply averaged (arithmetically averaged) to calculate the average thickness of the base layer 11. Note that the measurement points are randomly selected from the sample.

The base layer 11 includes, for example, a polyester as a main component. The polyester may be, for example, one of polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene terephthalate (PBT), polybutylene naphthalate (PBN), polycyclohexylenedimethylene terephthalate (PCT), polyethylene-p-oxybenzoate (PEB), and polyethylene bisphenoxycarboxylate, or a mixture of two or more thereof. Here, the “main component” means a component having the highest content ratio among the components constituting the base layer. For example, inclusion of a polyester in the base layer 11 as a main component may mean that the content ratio of the polyester in the base layer 11 is, for example, 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, 95% by mass or more, or 98% by mass or more with respect to the mass of the base layer 11, or may mean that the base layer 11 includes only a polyester.

In this embodiment, the base layer 11 may include a resin other than the polyester, described below, in addition to a polyester.

According to a preferred embodiment of the present technology, the base layer 11 may include PET or PEN.

In another embodiment of the present technology, the base layer 11 may include a resin other than a polyester. The resin forming the base layer 11 can contain, for example, at least one of a polyolefin-based resin, a cellulose derivative, a vinyl-based resin, or another polymer resin. In a case where the base layer 11 includes two or more of these resins, the two or more materials may be mixed, copolymerized, or laminated.

The polyolefin-based resin includes, for example, at least one of polyethylene (PE) or polypropylene (PP). The cellulose derivative includes, for example, at least one of cellulose diacetate, cellulose triacetate, cellulose acetate butyrate (CAB), or cellulose acetate propionate (CAP). The vinyl-based resin includes, for example, at least one of polyvinyl chloride (PVC) or polyvinylidene chloride (PVDC).

The other polymer resin includes, for example, at least one of polyether ether ketone (PEEK), polyamide or nylon (PA), aromatic polyamide or aramid (aromatic PA), polyimide (PI), aromatic polyimide (aromatic PI), polyamide imide (PAI), aromatic polyamide imide (aromatic PAI), polybenzoxazole (PBO) such as ZYLON (registered trademark), polyether, polyether ketone (PEK), polyether ester, polyether sulfone (PES), polyether imide (PEI), polysulfone (PSF), polyphenylene sulfide (PPS), polycarbonate (PC), polyarylate (PAR),or polyurethane (PU). According to a preferred embodiment of the present technology, the base layer 11 may include PEEK.

(Magnetic Layer)

The magnetic layer 13 can be, for example, a perpendicular recording layer. The magnetic layer 13 can include magnetic powder and a lubricant. The magnetic layer 13 may include, for example, a binder in addition to the magnetic powder and the lubricant, and may further include a binder and conductive particles, in particular. The magnetic layer 13 may further include an additive such as an abrasive or a rust preventive as necessary.

The magnetic layer 13 has pores. That is, the magnetic layer 13 has a surface having a large number of pores. Preferably, in the magnetic layer 13, pores are formed in an area in contact with a magnetic head in recording and/or reproduction of the magnetic recording medium 10, and particularly preferably, pores may be formed over the entire area. The pores may be opened perpendicularly to the surface of the magnetic layer 13.

The pores can be formed, for example, by pressing a large number of protrusions formed on the back layer side surface of the magnetic recording medium 10. In this case, the pores can correspond to the protrusions.

Note that in FIG. 1, the pores are indicated by reference numeral 13A, but FIG. 1 is a schematic diagram for better understanding of the present technology. The shapes of the pores 13A illustrated in FIG. 1 do not necessarily indicate the actual shapes.

The average thickness t_(m) of the magnetic layer 13 can satisfy preferably 35 nm≤t_(m)≤120 nm, more preferably 35 nm≤t_(m)≤100 nm, particularly preferably 35 nm≤t_(m)≤90 nm. The average thickness t_(m) of the magnetic layer 13 within the above numerical range contributes to improvement in electromagnetic conversion characteristics.

The average thickness t_(m) of the magnetic layer is particularly preferably 80 nm or less. The average thickness t_(m) of the magnetic layer 13 within this numerical range contributes to improvement in the recording/reproducing characteristics of the magnetic recording medium 10.

The average thickness t_(m) of the magnetic layer 13 is determined as follows, for example.

The magnetic recording medium 10 is processed to be thinned by a focused ion beam (FIB) method and the like. In a case where the FIB method is used, as a pretreatment for observing a TEM image of a cross section described later, a carbon film and a tungsten thin film are formed as protective films. The carbon film is formed on the magnetic layer side surface and the back layer side surface of the magnetic recording medium 10 by a vapor deposition method, and the tungsten thin film is further formed on the magnetic layer side surface by a vapor deposition method or a sputtering method. The thinning is performed in a length direction (longitudinal direction) of the magnetic recording medium 10. That is, by the thinning, a cross section parallel to both the longitudinal direction and the thickness direction of the magnetic recording medium 10 is formed.

The cross section of the obtained thinned sample is observed with a transmission electron microscope (TEM) under the following conditions to obtain a TEM image. Note that the magnification and the acceleration voltage may be appropriately adjusted according to the type of device.

Device: TEM (H9000NAR manufactured by Hitachi, Ltd.)

Acceleration voltage: 300 kV

Magnification: 100,000 times

Next, using the obtained TEM image, the thickness of the magnetic layer 13 is measured at 10 or more points in the longitudinal direction of the magnetic recording medium 10. An average value obtained by simply averaging (arithmetically averaging) the obtained measurement values is defined as the average thickness t_(m) [nm] of the magnetic layer 13. Note that the points where the measurement is performed are randomly selected from a test piece.

The magnetic layer 13 is preferably a perpendicularly oriented magnetic layer. Here, the perpendicular orientation means that a squareness ratio S1 measured in the longitudinal direction (traveling direction) of the magnetic recording medium 10 is 35% or less. A method for measuring the squareness ratio S1 will be separately described below.

Note that the magnetic layer 13 may be a magnetic layer that is in-plane oriented (longitudinally oriented). That is, the magnetic recording medium 10 may be a horizontal recording type magnetic recording medium. However, the perpendicular orientation is more preferable in terms of increasing the recording density.

(Servo Pattern)

A servo pattern is recorded on the magnetic layer 13. For example, as illustrated in FIG. 2A, the magnetic layer may have a plurality of servo bands SB and a plurality of data bands DB. The plurality of servo bands SB is disposed at regular intervals in a width direction of the magnetic recording medium 10. A data band DB is disposed between adjacent servo bands SB. In each of the servo bands SB, a servo signal for performing tracking control of a magnetic head may be written in advance. User data can be recorded in the data band DB.

The magnetic layer 13 can have, for example, at least one data band and at least two servo bands. The number of data bands can be, for example 2 to 10, particularly 3 to 6, and more particularly 4 or 5. The number of servo bands can be, for example, 3 to 11, particularly 4 to 7, and more particularly 5 or 6. These servo bands and data bands may be disposed, for example, so as to extend in the longitudinal direction of a tape-shaped magnetic recording medium (particularly, a long magnetic recording tape), in particular, so as to be in substantially parallel to each other. Examples of such a magnetic recording medium having a data band and a servo band include a magnetic recording tape according to the linear tape-open (LTO) standard. That is, the magnetic recording medium according to the present technology may be a magnetic recording tape according to the LTO standard. For example, the magnetic recording medium according to the present technology may be a magnetic recording tape according to LTO8 or a later standard.

A ratio R_(S)(=(S_(SB)/S)×100) of a total area S_(SB) of the servo bands SB with respect to an area S of the entire surface of the magnetic layer 13 is preferably 4.0% or less, more preferably 3.0% or less, and still more preferably 2.0% or less from a viewpoint of securing a high recording capacity.

Note that a servo bandwidth W_(SB) of each of the servo bands SB is preferably 95 μm or less, more preferably 60 μm or less, and still more preferably 30 μm or less from a viewpoint of securing a high recording capacity. The servo bandwidth W_(SB) is preferably 10 μm or more from a viewpoint of manufacturing a recording head.

The magnetic layer 13 can have, for example, five or more servo bands. The ratio R_(S) of the total area S_(SB) of the servo bands SB with respect to the area S of the surface of the magnetic layer 13 can be preferably 0.8% or more in order to secure five or more servo tracks.

The ratio R_(S) of the total area S_(SB) of the servo bands SB with respect to the area S of the entire surface of the magnetic layer 13 is determined as follows. For example, the magnetic recording medium 10 is developed using a ferricolloid developer (Sigmarker Q manufactured by Sigma Hi-Chemical Inc.), then the developed magnetic recording medium 10 is observed with an optical microscope, and the servo bandwidth W_(SB) and the number of servo bands SB are measured. Next, the ratio R_(S) is determined from the following formula.

Ratio R _(S)[%]=(((servo bandwidth W _(SB))×(number of servo bands))/(width of magnetic recording medium 10))×100

As illustrated in FIG. 2B, the magnetic layer 13 can form a plurality of data tracks Tk in a data band DB. In this case, a data track width W_(Tk) is preferably 2.0 μm or less, more preferably 1.5 μm or less, and still more preferably 1.0 μm or less from a viewpoint of securing a high recording capacity. The data track width W_(Tk) is preferably 0.02 μm or more from a viewpoint of a magnetic particle size. The data track width W_(Tk) is determined as follows. For example, a data recording pattern of a data band portion of the magnetic layer 13 on the entire surface of which data is recorded is observed using a magnetic force microscope (MFM) to obtain an MFM image. As the MFM, Dimension 3100 manufactured by Digital Instruments and analysis software thereof are used. A measurement area of the MFM image is set to 10 μm x 10 μm, and the measurement area of 10 μm×10 μm is divided into 512×512 (=262,144) measurement points. Measurement is performed with the MFM on three 10 μm×10 μm measurement areas at different locations, that is, three MFM images are obtained. From the three obtained MFM images, using the analysis software attached to Dimension 3100, the track width is measured at 10 locations and an average value (simple average) is taken. The average value is the data track width W_(Tk). Note that the MFM measurement conditions are: sweep speed: 1 Hz, chip used: MFMR-20, lift height: 20 nm, and correction: Flatten order 3.

The magnetic layer 13 can record data such that a minimum value of the distance L between magnetization inversions is preferably 48 nm or less, more preferably 44 nm or less, and still more preferably 40 nm or less from a viewpoint of securing a high recording capacity. The minimum value of the distance L between magnetization inversions is considered depending on a magnetic particle size. The minimum value of the distance L between magnetization inversions is determined as follows. For example, a data recording pattern of a data band portion of the magnetic layer 13 on the entire surface of which data is recorded is observed using a magnetic force microscope (MFM) to obtain an MFM image. As the MFM, Dimension 3100 manufactured by Digital Instruments and analysis software thereof are used. The measurement area of the MFM image is set to 2 μm×2 μm, and the measurement area of 2 μm×2 μm is divided into 512×512 (=262,144) measurement points. Measurement is performed with the MFM on three 2 μm×2 μm measurement areas at different locations, that is, three MFM images are obtained. 50 distances between bits are measured from a two-dimensional uneven chart of a recording pattern of the obtained MFM image. The distance between bits is measured using the analysis software attached to Dimension 3100. A value that is approximately the greatest common divisor of the measured 50 distances between bits is defined as the minimum value of the distance L between magnetization inversions. Note that the measurement conditions are: sweep speed: 1 Hz, chip used: MFMR-20, lift height: 20 nm, and correction: Flatten order 3.

More specific examples of the servo pattern recorded on the magnetic layer 13 of the magnetic recording medium of the present technology will be described below with reference to FIGS. 3 and 4. FIG. 3 is a schematic diagram of a data band and a servo band formed in the magnetic layer 13 of the magnetic recording medium 10. FIG. 4 is a diagram illustrating a servo pattern included in each servo band.

As illustrated in FIG. 3, the magnetic layer 13 has four data bands d0 to d3. The magnetic layer 13 has five servo bands s0 to s4 in total such that each data band is sandwiched between two servo bands.

As illustrated in FIG. 4, each servo band repeatedly has a frame unit (one servo frame) including five linear servo patterns inclined at a predetermined angle φ (for example, servo patterns A1 to A5), five linear servo patterns inclined at the same angle in the opposite direction to this signal (for example, servo patterns B1 to B5), four linear servo patterns inclined at a predetermined angle φ (for example, servo patterns C1 to C4), and four linear servo patterns inclined at the same angle in the opposite direction to this signal (for example, servo patterns D1 to D4). The predetermined angle φ can be, for example, 5° to 25°, and particularly 11° to 25°.

A servo bandwidth L1 (see FIG. 3) of each of the servo bands S0 to S4 may be, for example, 100 μm or less, particularly 60 μm or less, more particularly 50 μm or less, and further 40 μm or less. The servo bandwidth L1 may be, for example, 15 μm or more, and particularly 25 μm or more.

(Magnetic Powder)

Examples of a magnetic particle forming the magnetic powder included in the magnetic layer 13 include hexagonal ferrite, epsilon-type iron oxide (ϵ iron oxide), Co-containing spinel ferrite, gamma hematite, magnetite, chromium dioxide, cobalt-coated iron oxide, and metal, but are not limited thereto. The magnetic powder may be one of these or a combination of two or more thereof. Preferably, the magnetic powder can contain hexagonal ferrite, ϵ iron oxide, or Co-containing spinel ferrite. Particularly preferably, the magnetic powder is hexagonal ferrite. The hexagonal ferrite can particularly preferably contain at least one of Ba or Sr. The ϵ iron oxide can particularly preferably contain at least one of Al or Ga. These magnetic particles may be appropriately selected by those skilled in the art on the basis of factors such as a method for manufacturing the magnetic layer 13, the standard of the tape, and the function of the tape.

The shape of the magnetic particle depends on the crystal structure of the magnetic particle. For example, the barium ferrite (BaFe) and the strontium ferrite can each have a hexagonal plate shape. The ϵ iron oxide can be spherical. The cobalt ferrite can be cubic. The metal can be spindle-shaped. In a step of manufacturing the magnetic recording medium 10, these magnetic particles are oriented.

The average particle size of the magnetic powder can be preferably 50 nm or less, more preferably 40 nm or less, and still more preferably 30 nm or less, 25 nm or less, 22 nm or less, 21 nm or less, or 20 nm or less. The average particle size can be, for example, 10 nm or more, and preferably 12 nm or more.

The aspect ratio of the magnetic powder can be preferably 1.0 or more and 3.5 or less, more preferably 1.0 or more and 3.1 or less, still more preferably 1.0 or more and 2.8 or less, and particularly preferably 1.1 or more and 2.5 or less.

(Embodiment in which Magnetic Powder Contains Hexagonal Ferrite)

According to a preferred embodiment of the present technology, the magnetic powder can contain hexagonal ferrite, and more particularly can contain powder of nanoparticles containing hexagonal ferrite (hereinafter referred to as “hexagonal ferrite particles”). The hexagonal ferrite particle has, for example, a hexagonal plate shape or a substantially hexagonal plate shape. The hexagonal ferrite can preferably contain at least one of Ba, Sr, Pb, or Ca, more preferably at least one of Ba or Sr. Specifically, the hexagonal ferrite may be, for example, barium ferrite or strontium ferrite. The barium ferrite may further contain at least one of Sr, Pb, or Ca in addition to Ba. The strontium ferrite may further contain at least one of Ba, Pb, or Ca in addition to Sr.

More specifically, the hexagonal ferrite can have an average composition represented by a general formula MFe₁₂O₁₉. Here, M represents, for example, at least one metal of Ba, Sr, Pb, and Ca, preferably at least one metal of Ba and Sr. M may represent a combination of Ba and one or more metals selected from the group including Sr, Pb, and Ca. Furthermore, M may represent a combination of Sr and one or more metals selected from the group including Ba, Pb, and Ca. In the above general formula, some of Fe atoms may be replaced with another metal element.

In a case where the magnetic powder contains powder of hexagonal ferrite particles, the average particle size of the magnetic powder can be preferably 50 nm or less, more preferably 40 nm or less, and still more preferably 30 nm or less, 25 nm or less, 22 nm or less, 21 nm or less, or 20 nm or less. The average particle size can be, for example, 10 nm or more, preferably 12 nm or more, and more preferably 15 nm or more. For example, the average particle size of the magnetic powder can be 10 nm or more and 50 nm or less, 10 nm or more and 40 nm or less, 12 nm or more and 30 nm or less, 12 nm or more and 25 nm or less, or 15 nm or more and 22 nm or less. In a case where the average particle size of the magnetic powder is the upper limit value described above or less (for example, 50 nm or less, particularly 30 nm or less), in the magnetic recording medium 10 having a high recording density, good electromagnetic conversion characteristics (for example, SNR) can be obtained. In a case where the average particle size of the magnetic powder is the lower limit value described above or more (for example, 10 nm or more, preferably 12 nm or more), the dispersibility of the magnetic powder is further improved, and better electromagnetic conversion characteristics (for example, SNR) can be obtained.

In a case where the magnetic powder contains powder of hexagonal ferrite particles, the average aspect ratio of the magnetic powder can be preferably 1 or more and 3.5 or less, more preferably 1 or more and 3.1 or less, or 2 or more and 3.1 or less, and still more preferably 2 or more and 3 or less. When the average aspect ratio of the magnetic powder is within the above numerical range, aggregation of the magnetic powder can be suppressed, and moreover, resistance applied to the magnetic powder can be suppressed when the magnetic powder is perpendicularly oriented in a step of forming the magnetic layer 13. This can improve the perpendicular orientation of the magnetic powder.

In a case where the magnetic powder contains powder of hexagonal ferrite particles, the average particle size and average aspect ratio of the magnetic powder are determined as follows.

First, the magnetic recording medium 10 to be measured is processed to be thinned by a focused ion beam (FIB) method and the like. In a case where the FIB method is used, as a pretreatment for observing a TEM image of a cross section described later, a carbon film and a tungsten thin film are formed as protective films. The carbon film is formed on the magnetic layer side surface and the back layer side surface of the magnetic recording medium 10 by a vapor deposition method, and the tungsten thin film is further formed on the magnetic layer side surface by a vapor deposition method or a sputtering method. The thinning is performed in a length direction (longitudinal direction) of the magnetic recording medium 10. That is, by the thinning, a cross section parallel to both the longitudinal direction and the thickness direction of the magnetic recording medium 10 is formed.

Cross-sectional observation is performed for the cross section of the obtained thin sample such that the entire magnetic layer 13 is included with respect to the thickness direction of the magnetic layer 13 using a transmission electron microscope (H-9500 manufactured by Hitachi High-Technologies) with an acceleration voltage of 200 kV and an overall magnification of 500,000 times, and a TEM photograph is imaged.

Next, from the imaged TEM photograph, 50 particles which have side surfaces directed to an observation surface and the thicknesses of which can be clearly confirmed are selected. For example, FIG. 10 illustrates an example of the TEM photograph. In FIG. 10, for example, particles indicated by a and d are selected because their thicknesses can be clearly confirmed. A maximum plate thickness DA of each of the 50 selected particles is measured. The maximum plate thicknesses DA thus determined are simply averaged (arithmetically averaged) to determine an average maximum plate thickness DA_(ave).

Subsequently, the plate diameter DB of each particle of the magnetic powder is measured. In order to measure the particle plate diameter DB, 50 particles the plate diameters of which can be clearly confirmed are selected from the imaged TEM photograph. For example, in FIG. 10, for example, particles indicated by b and c are selected because their plate diameters can be clearly confirmed. The plate diameter DB of each of the 50 selected particles is measured. The plate diameters DB thus determined are simply averaged (arithmetically averaged) to determine an average plate diameter DB_(ave). The average plate diameter DB_(ave) is the average particle size.

Then, an average aspect ratio (DB_(ave)/DA_(ave)) of the particles is determined from the average maximum plate thickness DA_(ave) and the average plate diameter DB_(ave).

In a case where the magnetic powder contains powder of hexagonal ferrite particles, the average particle volume of the magnetic powder is preferably 5900 nm³ or less, more preferably 500 nm³ or more and 3400 nm³ or less, and still more preferably 1000 nm³ or more and 2500 nm³ or less.

In a case where the average particle volume of the magnetic powder is the upper limit value described above or less (for example, 5900 nm³ or less), good electromagnetic conversion characteristics (for example, SNR) can be obtained in the magnetic recording medium 10 having a high recording density. In a case where the average particle volume of the magnetic powder is the lower limit value described above or more (for example, 500 nm³ or more), the dispersibility of the magnetic powder is further improved, and better electromagnetic conversion characteristics (for example, SNR) can be obtained.

The average particle volume of the magnetic powder is determined as follows. First, the average maximum plate thickness DA_(ave) and the average plate diameter DB_(ave) are determined as described regarding the above-described method for calculating the average particle size of the magnetic powder. Next, an average volume V of the magnetic powder is determined by the following formula.

$V = {\frac{3\sqrt{3}}{8} \times {DA}_{ave} \times {DB}_{ave} \times {DB}_{ave}}$

According to a particularly preferred embodiment of the present technology, the magnetic powder can be barium ferrite magnetic powder or strontium ferrite magnetic powder, and more preferably barium ferrite magnetic powder. The barium ferrite magnetic powder includes iron oxide magnetic particles having barium ferrite as a main phase (hereinafter referred to as “barium ferrite particles”). The barium ferrite magnetic powder has high data recording reliability, for example, does not decrease a coercive force even in a high-temperature and high-humidity environment. The barium ferrite magnetic powder is preferable as the magnetic powder from such a viewpoint.

The average particle size of the barium ferrite magnetic powder is 50 nm or less, more preferably 10 nm or more and 40 nm or less, and still more preferably 12 nm or more and 25 nm or less.

In a case where the magnetic layer 13 contains barium ferrite magnetic powder as the magnetic powder, the average thickness t_(m) [nm] of the magnetic layer 13 preferably satisfies 35 nm≤t_(m)≤100 nm, and is particularly preferably 80 nm or less.

Furthermore, the coercive force Hc measured in a thickness direction (perpendicular direction) of the magnetic recording medium 10 is preferably 160 kA/m or more and 280 kA/m or less, more preferably 165 kA/m or more and 275 kA/m or less, and still more preferably 170 kA/m or more and 270 kA/m or less.

(Embodiment in which Magnetic Powder contains ϵ Iron Oxide)

According to another preferred embodiment of the present technology, the magnetic powder can preferably contain powder of nanoparticles containing ϵ iron oxide (hereinafter referred to as “ϵ iron oxide particles”). Even if the ϵ iron oxide particles are fine particles, a high coercive force can be obtained. ϵ iron oxide contained in the ϵ iron oxide particles is preferably crystal-oriented preferentially in a thickness direction (perpendicular direction) of the magnetic recording medium 10.

The ϵ iron oxide particle has a spherical shape or a substantially spherical shape, or has a cubic shape or a substantially cubic shape. Since the ϵ iron oxide particle has the shape as described above, in a case where the ϵ iron oxide particles are used as magnetic particles, a contact area between the particles in a thickness direction of the medium can be reduced, and aggregation of the particles can be suppressed as compared to a case where hexagonal plate-shaped barium ferrite particles are used as the magnetic particles. Therefore, dispersibility of the magnetic powder can be enhanced, and a better signal-to-noise ratio (SNR) can be obtained.

The ϵ iron oxide particle has a core-shell type structure. Specifically, the ϵ iron oxide particle has a core portion 21 and a two-layered shell portion 22 disposed around the core portion 21 as illustrated in FIG. 5. The two-layered shell portion 22 includes a first shell portion 22 a disposed on the core portion 21 and a second shell portion 22 b disposed on the first shell portion 22 a.

The core portion 21 contains ϵ iron oxide. ϵ iron oxide contained in the core portion 21 preferably contains an ϵ—Fe₂O₃ crystal as a main phase, and more preferably contains ϵ—Fe₂O₃ as a single phase.

The first shell portion 22 a covers at least a part of the periphery of the core portion 21. Specifically, the first shell portion 22 a may partially cover the periphery of the core portion 21 or may cover the entire periphery of the core portion 21. The first shell portion 22 a preferably covers the entire surface of the core portion 21 from a viewpoint of making exchange coupling between the core portion 21 and the first shell portion 22 a sufficient and improving magnetic characteristics.

The first shell portion 22 a is a so-called soft magnetic layer, and can include, for example, a soft magnetic material such as α—Fe, a Ni—Fe alloy, or a Fe—Si—Al alloy. α—Fe may be obtained by reducing ϵ iron oxide contained in the core portion 21.

The second shell portion 22 b is an oxide film as an antioxidant layer. The second shell portion 22 b contains a iron oxide, aluminum oxide, or silicon oxide. a-iron oxide can contain, for example, at least one iron oxide of Fe₃O₄, Fe₂O₃, and FeO. In a case where the first shell portion 22 a contains a-Fe (soft magnetic material), a-iron oxide may be obtained by oxidizing a-Fe contained in the first shell portion 22 a.

By inclusion of the first shell portion 22 a in the ϵ iron oxide particle as described above, thermal stability can be secured. As a result, the coercive force Hc of the entire iron oxide particles (core-shell particles) can be adjusted to a coercive force Hc suitable for recording while the coercive force Hc of the core portion 21 alone is maintained at a large value. Furthermore, by inclusion of the second shell portion 22 b in the ϵ iron oxide particle as described above, it is possible to suppress deterioration of the characteristics of the ϵ iron oxide particles due to generation of a rust or the like on surfaces of the particles by exposure of the ϵ iron oxide particles to the air during a step of manufacturing the magnetic recording medium 10 and before the step. Therefore, characteristic deterioration of the magnetic recording medium 10 can be suppressed.

The ϵ iron oxide particle may have a shell portion 23 having a single layer structure as illustrated in FIG. 6. In this case, the shell portion 23 has a similar configuration to the first shell portion 22 a. However, the ϵ iron oxide particle preferably has a two-layered shell portion 22 from a viewpoint of suppressing characteristic deterioration of the ϵ iron oxide particle.

The ϵ iron oxide particle may contain an additive instead of the core-shell structure, or may contain an additive while having the core-shell structure. In these cases, some of Fe atoms in the ϵ iron oxide particles are replaced with an additive. Even by inclusion of an additive in the ϵ iron oxide particle, the coercive force Hc of the entire ϵ iron oxide particles can be adjusted to a coercive force Hc suitable for recording. Therefore, recordability can be improved. The additive is a metal element other than iron, preferably a trivalent metal element, and more preferably at least one selected from the group including aluminum (Al), gallium (Ga), and indium (In).

Specifically, the ϵ iron oxide containing an additive is an ϵ—Fe₂—_(x)M_(x)O₃ crystal (in which M represents a metal element other than iron, preferably a trivalent metal element, and more preferably at least one of Al, Ga, or In, and x satisfies, for example, 0<x<1).

The average particle size (average maximum particle size) of the magnetic powder is preferably 22 nm or less, more preferably 8 nm or more and 22 nm or less, and still more preferably 12 nm or more and 22 nm or less. In the magnetic recording medium 10, an area having a half size of a recording wavelength is an actual magnetization area. Therefore, by setting the average particle size of the magnetic powder to a half or less of the shortest recording wavelength, it is possible to obtain good SNR. Therefore, when the average particle size of the magnetic powder is 22 nm or less, in the magnetic recording medium 10 having a high recording density (for example, the magnetic recording medium 10 that can record a signal at the shortest recording wavelength of 44 nm or less), good electromagnetic conversion characteristics (for example, SNR) can be obtained. Meanwhile, when the average particle size of the magnetic powder is 8 nm or more, dispersibility of the magnetic powder is further improved, and better electromagnetic conversion characteristics (for example, SNR) can be obtained.

The average aspect ratio of the magnetic powder is preferably 1.0 or more and 3.5 or less, more preferably 1.0 or more and 3.1 or less, and still more preferably 1.0 or more and 2.5 or less. When the average aspect ratio of the magnetic powder is within a range of 1 or more and 3.5 or less, aggregation of the magnetic powder can be suppressed, and resistance applied to the magnetic powder can be suppressed when the magnetic powder is perpendicularly oriented in a step of forming the magnetic layer 13. Therefore, perpendicular orientation of the magnetic powder can be improved.

In a case where, the magnetic powder contains ϵ iron oxide particles, the average particle size and average aspect ratio of the magnetic powder are determined as follows.

First, the magnetic recording medium 10 to be measured is processed to be thinned by a focused ion beam (FIB) method and the like. In a case where the FIB method is used, as a pretreatment for observing a TEM image of a cross section described later, a carbon film and a tungsten thin film are formed as protective films. The carbon film is formed on the magnetic layer side surface and the back layer side surface of the magnetic recording medium 10 by a vapor deposition method, and the tungsten thin film is further formed on the magnetic layer side surface by a vapor deposition method or a sputtering method. Thinning is performed in a length direction (longitudinal direction) of the magnetic recording medium 10. That is, by the thinning, a cross section parallel to both the longitudinal direction and the thickness direction of the magnetic recording medium 10 is formed.

Cross-sectional observation is performed for the cross section of the obtained thin sample such that the entire magnetic layer 13 is included with respect to the thickness direction of the magnetic layer 13 using a transmission electron microscope (H-9500 manufactured by Hitachi High-Technologies) with an acceleration voltage of 200 kV and an overall magnification of 500,000 times, and a TEM photograph is imaged.

Next, 50 particles the shapes of which can be clearly confirmed are selected from the imaged TEM photograph, and the long axis length DL and the short axis length DS of each of the particles are measured. Here, the long axis length DL means the largest distance among distances between two parallel lines drawn from all angles so as to come into contact with an outline of each of the particles (so-called maximum Feret diameter). Meanwhile, the short axis length DS means the largest length among the lengths of a particle in a direction orthogonal to the long axis (DL) of the particle.

Subsequently, the long axis lengths DL of the measured 50 particles are simply averaged (arithmetically averaged) to determine an average long axis length DL_(ave). The average long axis length DL_(ave) determined in this manner is taken as an average particle size of the magnetic powder. Furthermore, the short axis lengths DS of the measured 50 particles are simply averaged (arithmetically averaged) to determine an average short axis length DS_(ave). Then, an average aspect ratio (DL_(ave)/DS_(ave)) of the particles is determined from the average long axis length DL_(ave) and the average short axis length DS_(ave).

The average particle volume of the magnetic powder is preferably 5500 nm³ or less, more preferably 270 nm³ or more and 5500 nm³ or less, and still more preferably 900 nm³ or more and 5500 nm³ or less. When the average particle volume of the magnetic powder is 5500 nm³ or less, a similar effect to that in a case where the average particle size of the magnetic powder is 22 nm or less can be obtained. Meanwhile, when the average particle volume of the magnetic powder is 270 nm³ or more, a similar effect to a case where the average particle size of the magnetic powder is 8 nm or more can be obtained.

In a case where the ϵ iron oxide particle has a spherical shape or a substantially spherical shape, the average particle volume of the magnetic powder is determined as follows. First, an average long axis length DL_(ave) is determined in a similar manner to the above-described method for calculating the average particle size of the magnetic powder. Next, an average volume V of the magnetic powder is determined by the following formula.

V=(π/6)×DL _(ave) ³

In a case where the ϵ iron oxide particle has a cubic shape, the average volume of the magnetic powder is determined as follows.

The magnetic recording medium 10 is processed to be thinned by a focused ion beam (FIB) method and the like. In a case where the FIB method is used, as a pretreatment for observing a TEM image of a cross section described later, a carbon film and a tungsten thin film are formed as protective films. The carbon film is formed on the magnetic layer side surface and the back layer side surface of the magnetic recording medium 10 by a vapor deposition method, and the tungsten thin film is further formed on the magnetic layer side surface by a vapor deposition method or a sputtering method. The thinning is performed in a length direction (longitudinal direction) of the magnetic recording medium 10. That is, by the thinning, a cross section parallel to both the longitudinal direction and the thickness direction of the magnetic recording medium 10 is formed.

Cross-sectional observation is performed for the obtained thin sample such that the entire magnetic layer 13 is included with respect to the thickness direction of the magnetic layer 13 using a transmission electron microscope (H-9500 manufactured by Hitachi High-Technologies) with an acceleration voltage of 200 kV and an overall magnification of 500,000 times, and a TEM photograph is obtained. Note that the magnification and the acceleration voltage may be appropriately adjusted according to the type of device.

Next, 50 particles the shapes of which are clear are selected from the imaged TEM photograph, and the side length DC of each of the particles is measured. Subsequently, the side lengths DC of the measured 50 particles are simply averaged (arithmetically averaged) to determine an average side length DC_(ave). Next, the average volume V_(ave) (particle volume) of the magnetic powder is determined from the following formula using the average side length DC_(ave).

V _(ave) =DC _(ave) ³

(Embodiment in which magnetic powder contains Co-containing spinel ferrite)

According to still another preferred embodiment of the present technology, the magnetic powder can contain powder of nanoparticles containing Co-containing spinel ferrite (hereinafter also referred to as “cobalt ferrite particles”). That is, the magnetic powder can be cobalt ferrite magnetic powder. The cobalt ferrite particle preferably has uniaxial crystal anisotropy. The cobalt ferrite magnetic particle has, for example, a cubic shape or a substantially cubic shape. The Co-containing spinel ferrite may further contain at least one selected from the group including Ni, Mn, Al, Cu, and Zn in addition to Co.

Cobalt ferrite has, for example, an average composition represented by the following formula (1):

Co_(x)M_(y)Fe₂O_(z)  (1)

(Provided that in formula (1), M represents one or more metals selected from the group including Ni, Mn, Al, Cu, and Zn, for example. x represents a value within a range of 0.4≤x≤1.0. y represents a value within a range of 0≤y≤0.3. Provided that x and y satisfy a relationship of (x+y)≤1.0. z represents a value within a range of 3≤z≤4. Some of Fe atoms may be replaced with another metal element.)

The average particle size of the cobalt ferrite magnetic powder is preferably 25 nm or less, and more preferably 23 nm or less. The coercive force Hc of the cobalt ferrite magnetic powder is preferably 2500 Oe or more, and more preferably 2600 Oe or more and 3500 Oe or less.

In a case where the magnetic powder contains powder of cobalt ferrite particles, the average particle size of the magnetic powder is preferably 25 nm or less, and more preferably 10 nm or more and 23 nm or less. When the average particle size of the magnetic powder is 25 nm or less, good electromagnetic conversion characteristics (for example, SNR) can be obtained in the magnetic recording medium 10 having a high recording density. Meanwhile, when the average particle size of the magnetic powder is 10 nm or more, dispersibility of the magnetic powder is further improved, and better electromagnetic conversion characteristics (for example, SNR) can be obtained. In a case where the magnetic powder contains powder of cobalt ferrite particles, the average aspect ratio and average particle size of the magnetic powder are determined by the same method as that in a case where the magnetic powder contains ϵ iron oxide particles.

The average particle volume of the magnetic powder is preferably 15000 nm³ or less, and more preferably 1000 nm³ or more and 12000 nm³ or less. When the average particle volume of the magnetic powder is 15000 nm³ or less, a similar effect to that in a case where the average particle size of the magnetic powder is 25 nm or less can be obtained. Meanwhile, when the average particle volume of the magnetic powder is 1000 nm³ or more, a similar effect to a case where the average particle size of the magnetic powder is 10 nm or more can be obtained. Note that the average particle volume of the magnetic powder is determined by the same calculation method as that in a case where the iron oxide particle has a cubic shape.

(Lubricant)

The magnetic layer includes a lubricant. The lubricant may be, for example, one or more selected from fatty acids and/or fatty acid esters, and preferably includes both a fatty acid and a fatty acid ester. The fatty acid may preferably be a compound represented by the following general formula (1) or (2). For example, the lubricant may contain, as the fatty acid, one or both of a compound represented by the following general formula (1) and a compound represented by the general formula (2).

Furthermore, the fatty acid ester may preferably be a compound represented by the following general formula (3) or (4). For example, the lubricant may contain, as the fatty acid ester, one or both of a compound represented by the following general formula (3) and a compound represented by the general formula (4).

By inclusion of one or both of a compound represented by general formula (1) and a compound represented by general formula (2), and inclusion of one or both of a compound represented by general formula (3) and a compound represented by general formula (4) in the lubricant, an increase in the coefficient of dynamic friction due to repeated recording or reproduction in the magnetic recording medium can be suppressed.

CH₃(CH₂)_(k)COOH  (1)

(Provided that in the general formula (1), k is an integer selected from a range of 14 or more and 22 or less, more preferably a range of 14 or more and 18 or less.)

CH₃(CH₂)_(n)CH═CH(CH₂)_(m)COOH  (2)

(Provided that in the general formula (2), the sum of n and m is an integer selected from a range of 12 or more and 20 or less, more preferably a range of 14 or more and 18 or less.)

CH₃(CH₂)_(p)COO(CH₂)_(q)CH₃  (3)

(Provided that in the general formula (3), p is an integer selected from a range of 14 or more and 22 or less, more preferably a range of 14 or more and 18 or less, and q is an integer selected from a range of 2 or more and 5 or less, more preferably a range of 2 or more and 4 or less.)

CH₃(CH₂)_(r)COO—(CH₂)_(s)CH(CH₃)₂  (4)

(Provided that in the general formula (4), r is an integer selected from a range of 14 or more and 22 or less, and s is an integer selected from a range of 1 or more and 3 or less.)

(Binder)

As the binder, a resin having a structure in which a polyurethane-based resin, a vinyl chloride-based resin, or the like has been subjected to a crosslinking reaction is preferable. However, the binder is not limited to these resins, and other resins may be blended appropriately according to physical properties and the like required for the magnetic recording medium 10. Usually, a resin to be blended is not particularly limited as long as being generally used in the application type magnetic recording medium 10.

As the binder, for example, one or a combination of two or more selected from polyvinyl chloride, polyvinyl acetate, a vinyl chloride-vinyl acetate copolymer, a vinyl chloride-vinylidene chloride copolymer, a vinyl chloride-acrylonitrile copolymer, an acrylate-acrylonitrile copolymer, an acrylate-vinyl chloride-vinylidene chloride copolymer, an acrylate-vinylidene chloride copolymer, a methacrylate-vinylidene chloride copolymer, a methacrylate-vinyl chloride copolymer, a methacrylate-ethylene copolymer, polyvinyl fluoride, a vinylidene chloride-acrylonitrile copolymer, an acrylonitrile-butadiene copolymer, a polyamide resin, polyvinyl butyral, a cellulose derivative (cellulose acetate butyrate, cellulose diacetate, cellulose triacetate, cellulose propionate, and nitrocellulose), a styrene-butadiene copolymer, a polyester resin, an amino resin, and a synthetic rubber can be used.

Furthermore, a thermosetting resin or a reactive resin may be used as the binder. Examples of the thermosetting resin or the reactive resin include a phenol resin, an epoxy resin, a urea resin, a melamine resin, an alkyd resin, a silicone resin, a polyamine resin, and a urea formaldehyde resin.

Furthermore, in order to improve dispersibility of the magnetic powder, a polar functional group such as —SO₃M, —OSO₃M, —COOM, or P═O(OM)₂ may be introduced into each of the above-described binders. Here, in the formulae, M represents a hydrogen atom or an alkali metal such as lithium, potassium, or sodium.

Moreover, examples of the polar functional group include a side chain type group having a terminal group of —NR1R2 or —NR1R2R3⁺X⁻, and a main chain type group of >NR1R2⁺X⁻. Here, R1, R2, and R3 in the formula each independently represent a hydrogen atom or a hydrocarbon group, and X⁻ represents an ion of a halogen element such as fluorine, chlorine, bromine, or iodine, or an inorganic or organic ion. Furthermore, examples of the polar functional group include —OH, —SH, —CN, and an epoxy group.

(Additive)

As nonmagnetic reinforcing particles, the magnetic layer 13 may further contain aluminum oxide (a, (3, or y alumina), chromium oxide, silicon oxide, diamond, garnet, emery, boron nitride, titanium carbide, silicon carbide, titanium carbide, titanium oxide (rutile type or anatase type titanium oxide), and the like.

(Underlayer)

The underlayer 12 is a nonmagnetic layer containing nonmagnetic powder and a binder as main components. The underlayer 12 further contains a lubricant. The description regarding the binder and lubricant contained in the magnetic layer 13 described above also applies to the binder and lubricant contained in the underlayer 12. The underlayer 12 may further contain at least one additive selected from conductive particles, a curing agent, a rust preventive, and the like as necessary.

The average thickness of the underlayer 12 is preferably 0.6 μm or more and 2.0 μm or less, and more preferably 0.8 μm or more and 1.4 μm or less. Note that the average thickness of the underlayer 12 is determined in a similar manner to the average thickness t_(m) of the magnetic layer 13. However, a magnification of a TEM image is appropriately adjusted according to the thickness of the underlayer 12.

The underlayer 12 may have pores, that is, the underlayer 12 may have a large number of pores. The pores of the underlayer 12 may be formed, for example, along with formation of pores in the magnetic layer 13, and in particular, can be formed by pressing a large number of protrusions formed on the back layer side surface of the magnetic recording medium 10 against the magnetic layer side surface. That is, by forming a recess corresponding to the shape of a protrusion on the magnetic layer side surface, pores can be formed in the magnetic layer 13 and the underlayer 12.

Furthermore, pores may be formed as a solvent volatilizes in a step of drying a magnetic layer forming coating material. Furthermore, when the magnetic layer forming coating material is applied to a surface of the underlayer 12 in order to form the magnetic layer 13, a solvent in the magnetic layer forming coating material passes through the pores of the underlayer 12 formed when the lower layer is applied and dried, and can permeate the underlayer 12. Thereafter, when the solvent that has permeated the underlayer 12 volatilizes in a step of drying the magnetic layer 13, the solvent that has permeated the underlayer 12 moves from the underlayer 12 to the surface of the magnetic layer 13, thereby pores may be formed. The pores formed in this way can communicate, for example, the magnetic layer 13 with the underlayer 12. The average diameter of the pores can be adjusted by changing the solid content of the magnetic layer forming coating material or the type of a solvent thereof and/or drying conditions of the magnetic layer forming coating material.

By forming pores in both the magnetic layer 13 and the underlayer 12, a particularly suitable amount of lubricant for good traveling stability appears on the magnetic layer side surface, and an increase in the coefficient of dynamic friction due to repeated recording or reproduction can be further suppressed.

(Nonmagnetic Powder)

The nonmagnetic powder contained in the underlayer 12 can contain, for example, at least one selected from inorganic particles and organic particles. One kind of nonmagnetic powder may be used singly, or two or more kinds of nonmagnetic powder may be used in combination. The inorganic particles include, for example, one or a combination of two or more selected from a metal, a metal oxide, a metal carbonate, a metal sulfate, a metal nitride, a metal carbide, and a metal sulfide. More specifically, the inorganic particles can be one or more selected from, for example, iron oxyhydroxide, hematite, titanium oxide, and carbon black. Examples of the shape of the nonmagnetic powder include various shapes such as an acicular shape, a spherical shape, a cubic shape, and a plate shape, but are not limited thereto.

(Back Layer)

The back layer 14 can contain a binder and nonmagnetic powder. The back layer 14 may contain various additives such as a lubricant, a curing agent, and an antistatic agent as necessary. The above description regarding the binder and nonmagnetic powder contained in the underlayer 12 also applies to the binder and nonmagnetic powder contained in the back layer 14.

An average thickness t_(b) of the back layer 14 preferably satisfies t_(b)≤0.6 μm. By setting the average thickness t_(b) of the back layer 14 within the above range, even in a case where the average thickness t_(T) of the magnetic recording medium 10 satisfies t_(T)≤5.6 μm, the thicknesses of the underlayer 12 and the base layer 11 can be kept large. This makes it possible to maintain traveling stability of the magnetic recording medium 10 in the recording/reproducing device.

The average thickness t_(b) of the back layer 14 is determined as follows. First, the average thickness t_(T) of the magnetic recording medium 10 is measured. A method for measuring the average thickness t_(T) is as described in “(3) Physical properties and structure” below. Subsequently, the back layer 14 of the sample is removed with a solvent such as methyl ethyl ketone (MEK) or dilute hydrochloric acid. Next, the thickness of the sample is measured at five or more points using a laser hologage (LGH-110C) manufactured by Mitutoyo Corporation, and the measured values are simply averaged (arithmetically averaged) to calculate an average thickness t_(B) [μm]. Thereafter, the average thickness t_(b) (μm) of the back layer 14 is determined by the following formula. Note that the measurement points are randomly selected from the sample.

t _(b) [μm]=t _(T) [μm]−t _(B) [μm]

Out of the two surfaces of the back layer 14, the surface forming the back layer side surface of the magnetic recording medium 10 preferably has a large number of protrusions. By winding the magnetic recording medium 10 in a roll shape, the large

The protrusions can be formed, for example, by inclusion of particles in the back layer forming coating material. The particles can be inorganic particles such as carbon black, for example. The particle diameters of the particles can be appropriately selected according to the sizes of the pores to be formed in the magnetic layer 13.

The average particle size of particles (particularly inorganic particles) contained in the back layer 14 is preferably 10 nm or more and 300 nm or less, and more preferably 20 nm or more and 270 nm or less. The average particle size of the nonmagnetic powder is determined in a similar manner to the above-described average particle size of the magnetic powder. Furthermore, the nonmagnetic powder may have a particle size distribution of 2 or more.

(3) Physical Properties and Structure

(Number of Recesses having a Depth of 20% or more of the Thickness of Magnetic Layer)

The number of recesses having a depth of 20% or more of the thickness of the magnetic layer of the magnetic recording medium 10 is 10 or more and 200 or less per surface area of 1600 μm2. The number of the recesses may be preferably 12 or more, more preferably 14 or more, and still more preferably 15 or more. The number of the recesses may be preferably 180 or less, more preferably 160 or less, and still more preferably 140 or less or 120 or less. For example, the number of the recesses may be 12 or more and 160 or less, and can be more preferably 14 or more and 120 or less.

The number of recesses having a depth of 20% or more of the thickness of the magnetic layer is measured as follows.

The surface 13S of the magnetic layer 13 is observed by AFM, and an AFM image of 40 μm×40 μm is obtained. As the AFM, Dimension 3100, Nano Scope IIIa manufactured by Digital Instruments and an analysis software thereof are used. A cantilever including a silicon single crystal (SPM probe NCH normal type Point Probe L (cantilever length)=125 μm manufactured by Nano World) is used. Measurement is performed by tuning at 200 to 400 Hz as a tapping frequency. Next, the AFM image is divided into 512×512 (=262,144) measurement points. The height Z(i) (i: measurement point number, i=1 to 262,144) is measured at each measurement point. The measured heights Z(i) at the measurement points are simply averaged (arithmetically averaged) to determine an average height (reference plane) Z_(ave)(=Z(1)+Z(2)+ . . . +Z(262,144))/262,144). In this case, as image processing, data that has been subjected to filtering processing by Flatten order 2 and plane fit order 3 XY is used as data.

FIG. 13A is an example of the surface 13S of the magnetic layer 13 observed in an enlarged manner. In FIG. 13A, the XY plane is a direction in which the surface 13S of the magnetic layer 13 extends, and is an area having a surface area of, for example, 40 μm×40 μm=1600 μm². Furthermore, in FIG. 13A, the Z-axis indicates the depth of the recess 13A. By counting the number of recesses 13A having a depth from the reference plane corresponding to 20% or more of the average thickness (for example, 70 nm) of the magnetic layer 13 in an area having a surface area of 40 μm×40 μm=1600 μm², the number is determined. FIG. 13B schematically illustrates a distribution of the plurality of recesses 13A in the area having a surface area of 1600 μm² illustrated in FIG. 13A.

Specifically, a part of a cross section taken along the cut line IIB-IIB in FIG. 13A is illustrated. In FIG. 13B, the vertical axis corresponds to the depth of the recess 13A in the Z axis, and specifically indicates the ratio [%] of the depth of the recess 13A to the average thickness (for example, 70 nm) of the magnetic layer 13. In the cross section of FIG. 13B, the number of recesses 13A having a depth corresponding to 20% or more of the average thickness (for example, 70 nm) of the magnetic layer 13 is two, that is, recesses 13A-1 and 13A-2. FIG. 13C schematically illustrates a distribution of the plurality of recesses 13A in the area having a surface area of 1600 μm² illustrated in FIG. 13A. In the example illustrated in FIG. 13C, the number of recesses 13A having a depth corresponding to 20% or more of the average thickness (for example, 70 nm) of the magnetic layer 13 is 33. Note that the recess 13A illustrated in FIG. 13C corresponds to the recess 13A illustrated in FIG. 13A, and the recess 13A-1 and the recess 13A-2 illustrated in FIG. 13C correspond to the recess 13A-1 and the recess 13A-2 illustrated in FIG. 13B, respectively. Furthermore, as described later, the average thickness of the magnetic layer 13 is determined by thinly processing the magnetic recording medium 10 perpendicularly to a main surface thereof to manufacture a sample piece and observing a cross section of the test piece with a transmission electron microscope (TEM).

(Average Thickness t_(T) of Magnetic Recording Medium)

The average thickness t_(T) of the magnetic recording medium 10 satisfies t_(T)≤5.6 μm, and can be more preferably 5.3 μm or less, and still more preferably 5.2 μm or less, 5.0 μm or less, or 4.6 μm or less. When the average thickness t_(T) of the magnetic recording medium 10 is within the above numerical range (for example, by satisfying t_(T)≤5.6 μm), the recording capacity that can be recorded in one data cartridge can be increased as compared with a conventional case. A lower limit value of the average thickness t_(T) of the magnetic recording medium 10 is not particularly limited, but satisfies, for example, 3.5 μm≤t_(T).

The average thickness t_(T) of the magnetic recording medium 10 is determined as follows. First, the magnetic recording medium 10 having a width of ½ inches is prepared and cut into a length of 250 mm to manufacture a sample. Next, the thickness of the sample is measured at five or more points using a laser hologage (LGH-110C) manufactured by Mitutoyo Corporation as a measuring device, and the measured values are simply averaged (arithmetically averaged) to calculate an average thickness t_(T) [μm]. Note that the measurement points are randomly selected from the sample.

(Pore distribution of magnetic recording medium (pore volume and average pore diameter))

The average diameter of the pores of the magnetic recording medium 10 is 6 nm or more and 11 nm or less, preferably 6 nm or more and 10 nm or less, more preferably 6.5 nm or more and 10 nm or less, and still more preferably 7 nm or more and 9 nm or less in a state where the lubricant has been removed from the magnetic recording medium 10 and the magnetic recording medium 10 has been dried. The average pore diameter within the numerical range described above and the average pore diameter within the numerical range described above can suppress an increase in the coefficient of dynamic friction due to repeated recording or reproduction.

Furthermore, the pore volume of the magnetic recording medium 10 is preferably 0.016 cm³/g or more, and more preferably 0.02 cm³/g or more. The pore volume within the numerical range described above can further improve the effect of suppressing the increase in the coefficient of dynamic friction after repeated recording or reproduction is performed. The pore volume of the magnetic recording medium 10 can be, for example, 0.05 cm³/g or less, preferably 0.04 cm³/g or less, and more preferably 0.03 cm³/g or less.

The pore distribution (pore volume and average pore diameter (pore diameter of maximum pore volume at the time of desorption)) of the magnetic recording medium 10 is measured in a state where the lubricant has been removed from the magnetic recording medium and the magnetic recording medium has been dried. Specifically, the measurement is performed as follows.

First, the magnetic recording medium 10 having a size about 10% larger than the area 0.1265 m² is immersed in hexane (amount in which the tape can be sufficiently immersed, for example, 150 mL) for 24 hours, then naturally dried, and cut out so as to have an area of 0.1265 m² (for example, 50 cm at each end of the dried tape is cut off to prepare a tape having a width of 10 m) to manufacture a measurement sample. The lubricant is removed from the magnetic recording medium 10 by immersion in the hexane for 24 hours, and the magnetic recording medium 10 is dried by the natural drying.

Next, the pore distribution (pore volume and average pore diameter) is measured by a BJH method using a specific surface area/pore distribution measuring device. A measuring device and measuring conditions are indicated below. In this way, the pore volume and the average diameter of the pores are measured.

Measurement environment: room temperature

Measuring device: 3 FLEX manufactured by Micromeritics Instrument Corp.

Measurement adsorbate: N₂ gas

Measurement pressure range)(P/P⁰: 0 to 0.995

For the measurement pressure range, the pressure is changed as illustrated in Table below. The pressure values in the following Table are relative pressures P/P°. In the following Table, for example, in step 1, the pressure is changed so as to change by 0.001 per 10 seconds from a starting pressure 0.000 to an ultimate pressure 0.010. When the pressure reaches the ultimate pressure, pressure change in the next step is performed. The similar applies to steps 2 to 10. However, in each step, in a case where the pressure has not reached equilibrium, the device waits for the pressure to reach equilibrium and then proceeds to the next step.

Step Starting pressure Pressure change Ultimate pressure 1 0.000 0.001/10 sec  0.010 2 0.010 0.02/10 sec 0.100 3 0.100 0.05/10 sec 0.600 4 0.600 0.05/10 sec 0.950 5 0.950 0.05/10 sec 0.990 6 0.990 0.05/10 sec 0.995 7 0.995 0.01/10 sec 0.990 8 0.990 0.01/10 sec 0.950 9 0.950 0.05/10 sec 0.600 10 0.600 0.05/10 sec 0.300

(Kurtosis Sku of surface on back layer side)

The kurtosis Sku of the surface on the back layer side of the magnetic recording medium 10 can be 2.0 or more, preferably 2.1 or more, more preferably 2.2 or more, still more preferably 2.3 or more, and further still more preferably be 3.0 or more. Furthermore, the kurtosis Sku of the surface on the back layer side of the magnetic recording medium according to the present technology may be 4.4 or less, preferably 4.3 or less, and more preferably 4.2 or less.

The kurtosis of the surface on the magnetic layer side is measured as follows. First, the magnetic recording medium 10 having a width of 12.65 mm is prepared and cut into a length of 100 mm to manufacture a sample. Next, the sample is placed on a slide glass with a surface of the sample to be measured (surface on the magnetic layer side) upward, and an end of the sample is fixed with a mending tape. The shape of the surface is measured using Vert Scan (objective lens: 50 times) as a measuring device, and the kurtosis Sku is determined from the following formula on the basis of the ISO 25178 standard.

Device: non-contact roughness meter using optical interference

(Non-contact surface/layer cross-sectional shape measurement system Vert Scan R5500GL-M100-AC manufactured by Ryoka Systems Inc.)

Objective lens: 50 times

CCD: ⅓ lens

Measurement area: 640×480 pixels (field of view: about 95 μm×71 μm)

Measurement mode: phase

Wavelength filter: 520 nm

Noise reduction filter Smoothing 3×3

Surface correction: corrected with quadratic polynomial approximation surface

Measurement software: VS-Measure Version 5.5.2

Analysis software: VS-viewer Version 5.5.5

$S_{ku} = {{\frac{1}{S_{q}^{4}}\left\lbrack {\frac{1}{A}{\int{\int\limits_{A}{{z^{4}\left( {x,y} \right)}{dxdy}}}}} \right\rbrack}.}$

Here, meaning of each symbol in the formula is as follows.

$S_{q} = \sqrt{\frac{1}{A}{\int{\int\limits_{A}{{z^{4}\left( {x,y} \right)}{dxdy}}}}}$

A: Number of samples

x: Horizontal direction of sample

y: Vertical direction of sample

z: Height

As described above, the shape of the surface of the sample on the magnetic layer side is measured at five or more points in the longitudinal direction, and then an average value of the respective values of kurtosis Sku automatically calculated from surface profiles obtained at the respective points is taken as the kurtosis of the surface on the magnetic layer side.

(Friction Coefficient Ratio ((μ_(B)/μ_(A)))

The magnetic recording medium 10 has a friction coefficient ratio (μ_(B)/μ_(A)) of preferably 1.0 to 2.0, more preferably 1.0 to 1.8, still more preferably 1.0 to 1.6, in which μ_(A) represents a coefficient of dynamic friction between a magnetic layer side surface of the magnetic recording medium and a magnetic head in a state where a tension of 0.4 N is applied in the longitudinal direction of the magnetic recording medium 10, and μ_(B) represents a coefficient of dynamic friction between the magnetic layer side surface of the magnetic recording medium and the magnetic head in a state where a tension of 1.2 N is applied in the longitudinal direction of the magnetic recording medium. The friction coefficient ratio (μ_(B)/μ_(A)) within the above numerical range can suppress a change in the coefficient of dynamic friction due to the tension fluctuation during traveling, and therefore can stabilize traveling of the magnetic recording medium 10. The magnetic recording medium of the present technology capable of suppressing the change is suitable, for example, for use in a recording/reproducing device that can keep the width of a long magnetic recording medium constant or substantially constant by adjusting a tension of the magnetic recording medium in a longitudinal direction thereof.

The coefficients of dynamic friction μ_(A) and μ_(B) for calculating the friction coefficient ratio (μ_(B)/μ_(A)) are determined as follows.

First, as illustrated in FIG. 7(a), the magnetic recording medium 10 having a width of 1/2 inches is placed on two cylindrical guide rolls 73-1 and 73-2 each having a diameter of one inch and disposed in parallel to and spaced apart from each other such that the magnetic surface is in contact with the guide rolls 73-1 and 73-2. The two guide rolls 73-1 and 73-2 are fixed to a hard plate-shaped member 76, and thereby have a fixed positional relationship with each other.

Subsequently, the magnetic recording medium 10 is brought into contact with a head block (for recording/reproducing) 74 mounted on an LTO5 drive such that the magnetic surface is in contact with the head block 74 and a holding angle θ₁(°) is 5.6° . The head block 74 is disposed substantially at the center of the guide rolls 73-1 and 73-2.

The head block 74 is movably attached to the plate-shaped member 76 such that the holding angle θ₁ can be changed. When the holding angle θ₄(°) becomes 5.6°, the position of the head block 74 is fixed to the plate-shaped member 76. As a result, a positional relationship between the guide rolls 73-1 and 73-2 and the head block 74 is also fixed.

One end of the magnetic recording medium 10 is connected to a movable strain gauge 71 via a jig 72. The magnetic recording medium 10 is fixed to the jig 72 as illustrated in FIG. 7(b).

A weight 75 is connected to the other end of the magnetic recording medium 10. The weight 75 applies a tension T₀ [N] of 0.4 N to the magnetic recording medium 10 in a longitudinal direction thereof.

The movable strain gauge 71 is fixed onto a base 77. A positional relationship between the base 77 and the plate-shaped member 76 is also fixed. As a result, a positional relationship among the guide rolls 73-1 and 73-2, the head block 74, and the movable strain gauge 71 is fixed.

With the movable strain gauge 71, the magnetic recording medium 10 is slid on the head block 74 by 60 mm such that the magnetic recording medium 10 moves toward the movable strain gauge 71 at 10 mm/s. An output value (voltage) of the movable strain gauge 71 at the time of sliding is converted into T [N] on the basis of a linear relationship (as described later) between an output value acquired in advance and a load. T [N] is acquired 13 times from the start of sliding to the end of sliding for the 60 mm slide, and 11 values of T [N] excluding totally two times of the first and last times are simply averaged to obtain T_(ave) [N].

Thereafter, the coefficient of dynamic friction μ_(A) is determined by the following formula.

$\mu_{A} = {\frac{1}{\left( {\theta_{1}\lbrack{^\circ}\rbrack} \right) \times \left( {\Pi/180} \right)} \times {\ln\left( \frac{T_{ave}\lbrack N\rbrack}{T_{0}\lbrack N\rbrack} \right)}}$

The linear relationship is obtained as follows. That is, an output value (voltage) of the movable strain gauge 71 is obtained for each of cases where a load of 0.4 N is applied to the movable strain gauge 71 and a load of 1.5 N is applied thereto. From the obtained two output values and the two loads, a linear relationship between the output value and the load is obtained. Using the linear relationship, as described above, the output value (voltage) from the movable strain gauge 71 during sliding is converted into T [N].

The coefficient of dynamic friction μ_(B) is measured by the same method as the method for measuring the coefficient of dynamic friction μ_(A) except that the tension T₀ [N] applied to the other end is set to 1.2 N.

The friction coefficient ratio ((μ_(B)/μ_(A)) is calculated from the coefficients of dynamic friction μ_(A) and μ_(B) measured as described above.

(Friction Coefficient Ratio (μ_(C(1000))/μ_(C(5))))

The magnetic recording medium 10 has the friction coefficient ratio (μ_(C(1000))/μ_(C(5))) of preferably 1.0 to 2.0, more preferably 1.0 to 1.8, still more preferably 1.0 to 1.6, in which μ_(C(5)) represents a coefficient of dynamic friction at the fifth reciprocation in a case where the magnetic recording medium in a state where a tension of 0.6 N is applied to the magnetic recording medium in a longitudinal direction thereof is reciprocatedly slid five times on a magnetic head, and μ_(C(1000)) represents a coefficient of dynamic friction at the 1000th reciprocation in a case where the magnetic recording medium is reciprocated 1000 times on the magnetic head. The friction coefficient ratio (μ_(C(1000))/μ_(C(5))) within the above numerical range can reduce a change in the coefficient of dynamic friction due to traveling many times, and therefore can stabilize traveling of the magnetic recording medium 10.

The coefficients of dynamic friction μ_(C(5)) and μ_(C(1000)) for calculating the friction coefficient ratio (μ_(C(1000))/μ_(C(5))) are determined as follows.

The magnetic recording medium 10 is connected to the movable strain gauge 71 in the same manner as the method for measuring the coefficient of dynamic friction μ_(A) except that the tension T₀ [N] applied to the other end of the magnetic recording medium 10 is set to 0.6 N. Then, the magnetic recording medium 10 is slid 60 mm toward the movable strain gauge at 10 mm/s with respect to the head block 74 (forward path) and slid 60 mm away from the movable strain gauge (return path). This reciprocating operation is repeated 1000 times. Among the 1000 reciprocating operations, a strain gauge output value (voltage) is acquired 13 times from the start of sliding to the end of sliding for the 60 mm slide in the fifth forward path, and the output value is converted into T [N] on the basis of a linear relationship between an output value determined as described above for the coefficient of dynamic friction μ_(A) and a load. Eleven values of T [N] excluding totally two times of the first and last times are simply averaged to determine T_(ave) [N]. The coefficient of dynamic friction μ_(C(5)) is determined by the following formula.

$\mu_{C{(5)}} = {\frac{1}{\left( {{\theta_{1}\lbrack{^\circ}\rbrack} \times \left( {\pi/180} \right)} \right.} \times {\ln\left( \frac{T_{ave}\lbrack N\rbrack}{T_{0}\lbrack N\rbrack} \right)}}$

Moreover, the coefficient of dynamic friction μ_(C(1000)) is determined in a similar manner to the coefficient of dynamic friction μ_(C(5)) except that measurement is performed for the 1000th forward path.

The friction coefficient ratio μ_(C(1000)/μ_(C(5)) is calculated from the coefficients of dynamic friction μ_(C(5)) and μ_(C(1000)) measured as described above.

(Squareness ratio S2 measured in perpendicular direction)

The magnetic recording medium 10 has a squareness ratio S2 of preferably 65% or more, more preferably 70% or more, still more preferably 73% or more, further still more preferably 80% or more when the squareness ratio S2 is measured in a perpendicular direction (thickness direction) of the magnetic recording medium 10. When the squareness ratio S2 is 65% or more, perpendicular orientation of magnetic powder is sufficiently high. Therefore, better SNR can be obtained. Therefore, better electromagnetic conversion characteristics can be obtained. Furthermore, the shape of a servo signal is improved, and it is easier to control a drive side.

Here, the perpendicular orientation of the magnetic recording medium may mean that the squareness ratio S2 of the magnetic recording medium is within the numerical range described above (for example, 65% or more).

The squareness ratio S2 in the perpendicular direction is determined as follows. First, three magnetic recording media 10 are overlapped and bonded with a double-sided tape, and then punched with a φ6.39 mm punch to manufacture a measurement sample. At this time, marking is performed with an arbitrary ink having no magnetism such that the longitudinal direction (traveling direction) of the magnetic recording medium 10 be recognized. Then, using a VSM, an M-H loop of the measurement sample (the entire magnetic recording medium 10) corresponding to the perpendicular direction (thickness direction) of the magnetic recording medium 10 is measured. Next, the coating film (the underlayer 12, the magnetic layer 13, the back layer 14, and the like) is wiped off using acetone, ethanol, and the like, leaving only the base layer 11. Then, the three base layers 11 thus obtained are overlapped and bonded with a double-sided tape, and then punched with a φ6.39 mm punch to obtain a background correction sample (hereinafter simply referred to as “correction sample”). Thereafter, an M-H loop of the correction sample (base layer 11) corresponding to the perpendicular direction of the base layer 11 (perpendicular direction of the magnetic recording medium 10) is measured using a VSM.

In the measurement of the M-H loop of the measurement sample (the entire magnetic recording medium 10) and the M-H loop of the correction sample (base layer 11), a highly sensitive vibrating sample magnetometer “VSM-P7-15 type” manufactured by Toei Industry Co., Ltd. is used. The measurement conditions are set to measurement mode: full loop, maximum magnetic field: 15 kOe, magnetic field step: 40 bits, time constant of locking amp: 0.3 sec, waiting time: 1 sec, and MH average number: 20.

After the M-H loop of the measurement sample (the entire magnetic recording medium 10) and the M-H loop of the correction sample (base layer 11) are obtained, the M-H loop of the correction sample (base layer 11) is subtracted from the M-H loop of the measurement sample (the entire magnetic recording medium 10) to perform background correction, and an M-H loop after background correction is obtained. For the calculation of background correction, a measurement/analysis program attached to “VSM-P7-15 type” is used.

The squareness ratio S2 (%) is calculated by putting saturation magnetization Ms (emu) and residual magnetization Mr (emu) of the obtained M-H loop after background correction into the following formula. Note that each of the measurements of the M-H loops is performed at 25° C. Furthermore, when the M-H loop is measured in the perpendicular direction of the magnetic recording medium 10, “demagnetizing field correction” is not performed. Note that for this calculation, the measurement/analysis program attached to “VSM-P7-15” is used.

Squareness ratio S2 (%)=(Mr/Ms)×100

(Squareness Ratio S1 Measured In Longitudinal Direction)

The squareness ratio S1 measured in the longitudinal direction (traveling direction) of the magnetic recording medium 10 is preferably 35% or less, more preferably 30% or less, 27% or less, or 25% or less, and still more preferably 20% or less. When the squareness ratio S1 is 35% or less, perpendicular orientation of magnetic powder is sufficiently high. Therefore, better SNR can be obtained. Therefore, better electromagnetic conversion characteristics can be obtained. Furthermore, the shape of a servo signal is improved, and it is easier to control a drive side.

Here, the perpendicular orientation of the magnetic recording medium can mean that the squareness ratio S1 of the magnetic recording medium is within the numerical range described above (for example, 35% or less). The magnetic recording medium according to the present technology is preferably perpendicularly oriented.

The squareness ratio S1 in the longitudinal direction is determined in a similar manner to the squareness ratio S2 except that the M-H loop is measured in the longitudinal direction (traveling direction) of the magnetic recording medium 10 and the base layer 11.

The squareness ratios S1 and S2 are set to desired values, for example, by adjusting the intensity of a magnetic field applied to a magnetic layer forming coating material, application time of the magnetic field to the magnetic layer forming coating material, a dispersed state of magnetic powder in the magnetic layer forming coating material, and the concentration of a solid content in the magnetic layer forming coating material. Specifically, for example, as the intensity of the magnetic field is increased, the squareness ratio S1 becomes smaller, whereas the squareness ratio S2 becomes larger. Furthermore, as the application time of the magnetic field is increased, the squareness ratio S1 becomes smaller, whereas the squareness ratio S2 becomes larger. Furthermore, as the dispersed state of the magnetic powder is improved, the squareness ratio S1 becomes smaller, whereas the squareness ratio S2 becomes larger. Furthermore, as the concentration of the solid content decreases, the squareness ratio S1 becomes smaller, whereas the squareness ratio S2 becomes larger. Note that the adjustment methods may be used singly or in combination of two or more thereof.

(Arithmetic Average Roughness R_(a))

The arithmetic average roughness R_(a) of the magnetic layer side surface (hereinafter also referred to as “magnetic surface”) of the magnetic recording medium 10 may be preferably 2.5 nm or less, more preferably 2.0 nm or less, and still more preferably 1.8 nm or less or 1.7 nm or less. When R_(a) is within this numerical range, a better SNR can be obtained.

The arithmetic average roughness R_(a) is determined as follows. First, the surface of the magnetic layer 13 is observed by AFM, and an AFM image of 40 μm×40 μm is obtained. As the AFM, Dimension 3100 manufactured by Digital Instruments and an analysis software thereof are used. A cantilever including a silicon single crystal is used (Note 1). Measurement is performed by tuning at 200 to 400 Hz as a tapping frequency.

Next, an AFM image is divided into 512×512 (=262,144) measurement points. The height Z(i) (i: measurement point number, i=1 to 262,144) is measured at each measurement point. The measured heights Z(i) at the measurement points are simply averaged (arithmetically averaged) to determine average height (average plane) Z_(ave)(=Z(1)+Z(2)+ . . . +Z(262,144))/262,144). Subsequently, a deviation Z″(i) (=|Z(i)−Z_(ave)|) from an average center line at each measurement point is determined, and the arithmetic average roughness R_(a) [nm](=(Z″(1)+Z″(2)+ . . . +Z″(262,144))/262,144) is calculated. In this case, as image processing, data that has been subjected to filtering processing by Flatten order 2 and plane fit order 3 XY is used as data.

(Note 1) SPM probe NCH normal type Point Probe L (cantilever length)=125 μm manufactured by Nano World

(Coercive Force Hc)

The coercive force Hc of the magnetic recording medium 10 in a longitudinal direction thereof is preferably 2000 Oe or less, more preferably 1900 Oe or less, and still more preferably 1800 Oe or less. When the coercive force Hc in the longitudinal direction is 2000 Oe or less, magnetization reacts with high sensitivity due to a magnetic field in a perpendicular direction from a recording head. Therefore, a good recording pattern can be formed.

The coercive force Hc measured in the longitudinal direction of the magnetic recording medium 10 is preferably 1000 Oe or more. When the coercive force Hc in the longitudinal direction is 1000 Oe or more, demagnetization due to a leakage magnetic flux from a recording head can be suppressed.

The above coercive force Hc is determined as follows. First, three magnetic recording media 10 are overlapped and bonded with a double-sided tape, and then punched with a φ6.39 mm punch to manufacture a measurement sample. At this time, marking is performed with an arbitrary ink having no magnetism such that the longitudinal direction (traveling direction) of the magnetic recording medium 10 be recognized. Then, an M-H loop of the measurement sample (the entire magnetic recording medium 10) corresponding to the longitudinal direction (traveling direction) of the magnetic recording medium 10 is measured using a vibrating sample magnetometer (VSM). Next, the coating film (the underlayer 12, the magnetic layer 13, the back layer 14, and the like) is wiped off using acetone, ethanol, and the like, leaving only the base layer 11. Then, the three base layers 11 thus obtained are overlapped and bonded with a double-sided tape, and then punched with a φ6.39 mm punch to manufacture a background correction sample (hereinafter simply referred to as “correction sample”). Thereafter, an M-H loop of the correction sample (base layer 11) corresponding to the perpendicular direction of the base layer 11 (perpendicular direction of the magnetic recording medium 10) is measured using a VSM.

In the measurement of the M-H loop of the measurement sample (the entire magnetic recording medium 10) and the M-H loop of the correction sample (base layer 11), a highly sensitive vibrating sample magnetometer “VSM-P7-15 type” manufactured by Toei Industry Co., Ltd. is used. The measurement conditions are set to measurement mode: full loop, maximum magnetic field: 15 kOe, magnetic field step: 40 bits, time constant of locking amp: 0.3 sec, waiting time: 1 sec, and MH average number: 20.

After the M-H loop of the measurement sample (the entire magnetic recording medium 10) and the M-H loop of the correction sample (base layer 11) are obtained, the M-H loop of the correction sample (base layer 11) is subtracted from the M-H loop of the measurement sample (the entire magnetic recording medium 10) to perform background correction, and an M-H loop after background correction is obtained. For the calculation of background correction, a measurement/analysis program attached to “VSM-P7-15 type” is used.

The coercive force Hc is determined from the obtained M-H loop after background correction. Note that for this calculation, the measurement/analysis program attached to “VSM-P7-15” is used. Note that each of the measurements of the M-H loops is performed at 25° C. Furthermore, when the M-H loop is measured in the longitudinal direction of the magnetic recording medium 10, “demagnetizing field correction” is not performed.

(Young's Modulus of Magnetic Recording Medium)

The Young's modulus of the magnetic recording medium 10 in a longitudinal direction thereof is 7.90 GPa or less, more preferably 7.85 or less, and still more preferably 7.80 GPa or less. The above Young's modulus may be, for example, 3.00 GPa or more, preferably 4.00 Gpa or more, more preferably 5.00 Gpa or more, still more preferably 6.00 GPa or more, and further still more preferably 7.00 GPa or more. Since the Young's modulus is within the numerical range described above, the magnetic recording medium 10 is suitable for use in a recording/reproducing device that keeps the width of the magnetic recording medium constant or substantially constant by adjusting a tension of the magnetic recording medium 10 in a longitudinal direction thereof. The recording/reproducing device can detect, for example, dimensions or a dimensional change of the magnetic recording medium in a width direction thereof, and can adjust the tension in the longitudinal direction on the basis of a detection result.

The Young's modulus of the magnetic recording medium 10 in a longitudinal direction thereof can be measured as follows.

The Young's modulus described above is measured using a tensile tester (AG-100D manufactured by Shimadzu Corporation). First, the magnetic recording medium 10 having a width of ½ inches is cut into a length of 180 mm to prepare a measurement sample.

Two jigs that can fix the measurement sample so as to cover the entire width thereof are attached to the above tensile tester. Two ends in the length direction of the measurement sample are chucked by the two jigs, respectively. A distance between the chucks is set to 100 mm. After the measurement sample is chucked, a stress is gradually applied such that the measurement sample is pulled in the length direction. The pulling speed is set to 0.1 mm/min. From the change in stress and the amount of elongation at this time, the Young's modulus is calculated using the following formula.

E={ΔN/S)/(Δx/L}×10⁶

In the formula described above, ϵ represents Young's modulus (N/m²), ΔN represents a change in stress (N), S represents the cross-sectional area of a measurement sample (mm²), Δx represents the amount of elongation (mm), and L represents a distance between the two jigs (distance between chucks) (mm).

The stress when the measurement sample is pulled by the tensile tester described above is changed from 0.5 N to 1.0 N. The change in stress (ΔN) and the amount of elongation (Δx) when the stress is changed in this way are used for the calculation by the formula described above.

(4) Method for Manufacturing Magnetic Recording Medium

Next, a method for manufacturing the magnetic recording medium 10 having the above-described configuration will be described. First, by kneading and/or dispersing nonmagnetic powder, a binder, and the like in a solvent, an underlayer forming coating material is prepared. Next, by kneading and/or dispersing magnetic powder, a binder, and the like in a solvent, a magnetic layer forming coating material is prepared. Next, by kneading and/or dispersing a binder, nonmagnetic powder, and the like in a solvent, a back layer forming coating material is prepared. For the preparation of the magnetic layer forming coating material, the underlayer forming coating material, and the back layer forming coating material, for example, the following solvents, dispersing devices, and kneading devices can be used.

Examples of the solvent used for preparing the above-described coating material include: a ketone-based solvent such as acetone, methyl ethyl ketone, methyl isobutyl ketone, or cyclohexanone; an alcohol-based solvent such as methanol, ethanol, or propanol; an ester-based solvent such as methyl acetate, ethyl acetate, butyl acetate, propyl acetate, ethyl lactate, or ethylene glycol acetate; an ether-based solvent such as diethylene glycol dimethyl ether, 2-ethoxyethanol, tetrahydrofuran, or dioxane; an aromatic hydrocarbon-based solvent such as benzene, toluene, or xylene; and a halogenated hydrocarbon-based solvent such as methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, or chlorobenzene. One of these solvents may be used, or a mixture of two or more thereof may be used.

Examples of a kneading device used for preparing the above-described coating materials include a kneading device such as a continuous twin-screw kneading machine, a continuous twin-screw kneading machine capable of performing dilution in multiple stages, a kneader, a pressure kneader, or a roll kneader, but are not particularly limited to these devices. Furthermore, examples of a dispersing device used for preparing the above-described coating materials include a dispersing device such as a roll mill, a ball mill, a horizontal sand mill, a vertical sand mill, a spike mill, a pin mill, a tower mill, a pearl mill (for example, “DCP mill” manufactured by Eirich Co., Ltd.), a homogenizer, or an ultrasonic wave dispersing machine, but are not particularly limited to these devices.

Next, the underlayer forming coating material is applied to one main surface of the base layer 11 and dried to form the underlayer 12. Subsequently, by applying the magnetic layer forming coating material onto the underlayer 12 and drying the magnetic layer forming coating material, the magnetic layer 13 is formed on the underlayer 12. Note that during drying, magnetic powder is subjected to magnetic field orientation in a thickness direction of the base layer 11 by, for example, a solenoid coil. Furthermore, during drying, the magnetic powder may be subjected to magnetic field orientation in a longitudinal direction (traveling direction) of the base layer 11 by, for example, a solenoid coil, and then may be subjected to magnetic field orientation in a thickness direction of the base layer 11. By such a magnetic field orientation treatment, the perpendicular orientation degree (that is, squareness ratio S1) of the magnetic powder can be improved. After the magnetic layer 13 is formed, by applying the back layer forming coating material to the other main surface of the base layer 11 and drying the back layer forming coating material, the back layer 14 is formed. As a result, the magnetic recording medium 10 is obtained.

The squareness ratios S1 and S2 can be set to desired values, for example, by adjusting the intensity of a magnetic field applied to a coating film of the magnetic layer forming coating material, adjusting the concentration of a solid content in the magnetic layer forming coating material, or adjusting drying conditions (for example, drying temperature and drying time) of a coating film of the magnetic layer forming coating material. The intensity of a magnetic field applied to the coating film is preferably 2 to 3 times the coercive force of the magnetic powder. In order to further increase the squareness ratio S1 (that is, to further reduce the squareness ratio S2), it is preferable to improve the dispersion state of the magnetic powder in the magnetic layer forming coating material. Furthermore, in order to further increase the squareness ratio S1, it is also effective to magnetize the magnetic powder before the magnetic layer forming coating material is put into an orientation device for magnetic field orientation of the magnetic powder. Note that the above methods for adjusting the squareness ratios S1 and S2 may be used singly or in combination of two or more thereof.

Thereafter, the obtained magnetic recording medium 10 is calendered to smooth the surface of the magnetic layer 13. Next, the magnetic recording medium 10 that has been calendered is wound into a roll shape. Thereafter, the magnetic recording medium 10 is heated in this state, and the large number of protrusions 14A on the surface of the back layer 14 are thereby transferred onto the surface of the magnetic layer 13. As a result, pores (a large number of holes 13A) are formed on the surface of the magnetic layer 13.

The temperature of the heat treatment is preferably 55° C. or higher and 75° C. or lower. By adopting a temperature within this numerical range as the temperature of the heat treatment, the shape of the protrusion is transferred onto the magnetic layer 13 satisfactorily. In a case where the temperature of the heat treatment is too low (for example, less than 55° C.), the shape of the protrusion is not be sufficiently transferred in some cases. In a case where the temperature of the heat treatment is too high (for example, higher than 75° C.), the amount of pores may be excessively increased, and the lubricant on the surface of the magnetic layer 13 may be excessive. Here, the temperature of the heat treatment is the temperature of an atmosphere holding the magnetic recording medium 10.

The time for the heat treatment is preferably 15 hours or more and 40 hours or less. By setting the time for the heat treatment within this numerical range, the shape of the protrusion is transferred onto the magnetic layer 13 satisfactorily. In a case where the time for the heat treatment is too short (for example, less than 15 hours), the shape of the protrusion is not be sufficiently transferred in some cases. In order to suppress a decrease in productivity, the time for the heat treatment is desirably set to 40 hours or less, for example.

Finally, the magnetic recording medium 10 is cut into a predetermined width (for example, a width of ½ inches). The target magnetic recording medium 10 is thereby obtained. A servo pattern is recorded on the magnetic recording medium 10. A servo pattern may be recorded, for example, by a servo writer known in the present technical field.

In the above manufacturing method, the large number of protrusions 14A formed on the surface of the back layer 14 are transferred onto the surface of the magnetic layer 13, and pores are thereby formed on the surface of the magnetic layer 13. However, the method for forming the pores is not limited thereto. For example, pores may be formed on the surface of the magnetic layer 13 by adjusting the type of a solvent included in the magnetic layer forming coating material and/or adjusting drying conditions of the magnetic layer forming coating material. Furthermore, for example, in the process of drying the solvent of the magnetic layer forming coating material, pores can be formed by an uneven distribution of the solid and the solvent included in the magnetic layer forming coating material. Furthermore, in the process of applying the magnetic layer forming coating material, the solvent included in the magnetic layer forming coating material can also be absorbed by the underlayer 12 through the pores of the underlayer 12 formed when the lower layer is applied and dried. In the drying step after the application, the solvent moves from the underlayer 12 through the magnetic layer 13, and pores connecting the magnetic layer 13 to the underlayer 12 can be thereby formed.

(5) Recording/Reproducing Device

[Configuration of Recording/Reproducing Device]

Next, an example of the configuration of the recording/reproducing device 30 for performing recording and reproduction for the magnetic recording medium 10 having the configuration described above will be described with reference to FIG. 8.

The recording/reproducing device 30 can adjust a tension applied to the magnetic recording medium 10 in a longitudinal direction thereof. Furthermore, the recording/reproducing device 30 can load the magnetic recording cartridge 10A thereon. Here, a case where the recording/reproducing device 30 can load one magnetic recording cartridge 10A thereon will be described in order to facilitate the description. However, the recording/reproducing device 30 can load a plurality of the magnetic recording cartridges 10A thereon.

The recording/reproducing device 30 is connected to information processing devices such as a server 41 and a personal computer (hereinafter referred to as “PC”) 42 through a network 43, and data supplied from these information processing devices can be recorded in the magnetic recording cartridge 10A. The shortest recording wavelength of the recording/reproducing device 30 is preferably 100 nm or less, more preferably 75 nm or less, still more preferably 60 nm or less, and particularly preferably 50 nm or less.

As illustrated in FIG. 8, the recording/reproducing device 30 includes a spindle 31, a reel 32 on the recording/reproducing device side, a spindle driving device 33, a reel driving device 34, a plurality of guide rollers 35, a head unit 36, a communication interface (hereinafter, I/F) 37, and a control device 38.

The spindle 31 can mount the magnetic recording cartridge 10A thereon. The magnetic recording cartridge 10A conforms to the linear tape open (LTO) standard, and rotatably houses a single reel 10C around which the magnetic recording 10 is wound in a cartridge case 10B. A V-shaped servo pattern is recorded in advance as a servo signal on the magnetic recording medium 10. The reel 32 can fix a tip of the magnetic recording medium 10 pulled out from the magnetic recording cartridge 10A.

The spindle driving device 33 is a device that rotationally drives the spindle 31. The reel driving device 34 is a device that rotationally drives the reel 32. When data is recorded or reproduced on the magnetic recording medium 10, the spindle driving device 33 and the reel driving device 34 rotationally drive the spindle 31 and the reel 32 to cause the magnetic recording medium 10 to travel. The guide roller 35 is a roller for guiding travel of the magnetic recording medium 10.

The head unit 36 includes a plurality of recording heads for recording a data signal on the magnetic recording medium 10, a plurality of reproducing heads for reproducing a data signal recorded on the magnetic recording medium 10, and a plurality of servo heads for reproducing a servo signal recorded on the magnetic recording medium 10. For example, a ring type head can be used as the recording head, but the type of the recording head is not limited thereto.

The communication I/F 37 is for communicating with an information processing device such as the server 41 or the PC 42, and is connected to the network 43.

The control device 38 controls the entire recording/reproducing device 30. For example, the control device 38 causes the head unit 36 to record a data signal supplied from an information processing device such as the server 41 or the PC 42 on the magnetic recording medium 10 in response to a request from the information processing device. Furthermore, the control device 38 causes the head unit 36 to reproduce a data signal recorded on the magnetic recording medium 10 in response to a request from an information processing device such as the server 41 or the PC 42 and supplies the data signal to the information processing device.

[Operation of Recording/Reproducing Device]

Next, operation of the recording/reproducing device 30 having the configuration described above will be described.

First, the magnetic recording cartridge 10A is mounted on the recording/reproducing device 30. A tip of the magnetic recording medium 10 is pulled out, transferred to the reel 32 via the plurality of guide rollers 35 and the head unit 36, and attached to the reel 32.

Next, when an operation unit (not illustrated) is operated, the spindle driving device 33 and the reel driving device 34 are driven by control of the control device 38, and the spindle 31 and the reel 32 are rotated in the same direction such that the magnetic recording medium 10 travels from the reel 10C toward the reel 32. As a result, while the magnetic recording medium 10 is wound around the reel 32, the head unit 36 records information on the magnetic recording medium 10 or reproduces information recorded on the magnetic recording medium 10.

Furthermore, in a case where the magnetic recording medium 10 is rewound onto the reel 10C, the spindle 31 and the reel 32 are rotationally driven in the opposite direction to the direction described above, and the magnetic recording medium 10 thereby travels from the reel 32 to the reel 10C. Also during the rewinding, the head unit 36 records information on the magnetic recording medium 10 or reproduces information recorded on the magnetic recording medium 10.

(6) Cartridge

[Configuration of Cartridge]

The present technology also provides a magnetic recording cartridge (also referred to as a tape cartridge) including the magnetic recording medium according to the present technology. In the magnetic recording cartridge, the magnetic recording medium may be wound around a reel, for example. For example, the magnetic recording cartridge may include: a communication unit that communicates with a recording/reproducing device; a storage unit; and a control unit that stores information received from the recording/reproducing device through the communication unit in the storage unit, reads the information from the storage unit according to a request from the recording/reproducing device, and transmits the information to the recording/reproducing device through the communication unit. The information can include adjustment information for adjusting a tension applied to the magnetic recording medium in a longitudinal direction thereof. The adjustment information can include, for example, dimensional information in the width direction at a plurality of positions in the longitudinal direction of the magnetic recording medium. The dimensional information in the width direction may be dimensional information at the time of manufacturing a magnetic recording medium (initial stage after manufacture) described below in [Configuration of cartridge memory] and/or dimensional information acquired in recording and/or reproducing the magnetic recording medium.

An example of the configuration of the cartridge 10A including the magnetic recording medium 10 having the above-described configuration will be described with reference to FIG. 11.

FIG. 11 is an exploded perspective view illustrating an example of the configuration of the cartridge 10A. The cartridge 10A is a magnetic recording medium cartridge conforming to the linear tape-open (LTO) standard, and includes: in the cartridge case 10B including a lower shell 212A and an upper shell 212B, a reel 10C around which a magnetic tape (tape-shaped magnetic recording medium) 10 is wound; a reel lock 214 and a reel spring 215 for locking rotation of the reel 10C; a spider 216 for releasing a locked state of the reel 10C; a slide door 217 that opens and closes a tape outlet 212C formed in the cartridge case 10B so as to straddle the lower shell 212A and the upper shell 212B; a door spring 218 that urges the slide door 217 to a closed position of the tape outlet 212C; a write protect 219 for preventing erroneous erasure; and a cartridge memory 211. The reel 10C has a substantially disk shape with an opening at the center, and incudes a reel hub 213A and a flange 213B including a hard material such as plastic. A leader pin 220 is disposed at one end of the magnetic tape 10.

The cartridge memory 211 is disposed near one corner of the cartridge 10A. The cartridge memory 211 faces a reader/writer (not illustrated) of the recording/reproducing device 30 in a state where the cartridge 10A is loaded on the recording/reproducing device 30. The cartridge memory 211 communicates with the recording/reproducing device 30, specifically, with a reader/writer (not illustrated) according to a wireless communication standard conforming to the LTO standard.

[Configuration of Cartridge Memory]

An example of the configuration of the cartridge memory 211 will be described with reference to FIG. 12.

FIG. 12 is a block diagram illustrating an example of the configuration of the cartridge memory 211. The cartridge memory 211 includes: an antenna coil (communication unit) 331 that communicates with a reader/writer (not illustrated) according to a prescribed communication standard; a rectification/power supply circuit 332 that generates power using an induced electromotive force from a radio wave received by an antenna coil 331 and performs rectification to generate a power supply; a clock circuit 333 that generates a clock using an induced electromotive force similarly from the radio wave received by the antenna coil 331; a detection/modulation circuit 334 that performs detection of the radio wave received by the antenna coil 331 and modulation of a signal transmitted by the antenna coil 331; a controller (control unit) 335 including a logic circuit and the like for determining a command and data from a digital signal extracted from the detection/modulation circuit 334 and processing the command and data; and a memory (storage unit) 336 that stores information. Furthermore, the cartridge memory 211 includes a capacitor 337 connected in parallel to the antenna coil 331, and the antenna coil 331 and the capacitor 337 constitute a resonant circuit.

The memory 336 stores information and the like related to the cartridge 10A. The memory 336 is a non-volatile memory (NVM). The memory 336 preferably has a storage capacity of about 32 KB or more. For example, in a case where the cartridge 10A conforms to the LTO-9 standard or the LTO-10 standard, the memory 336 has a storage capacity of about 32 KB.

The memory 336 has a first storage area 336A and a second storage area 336B. The first storage area 336A corresponds to a storage area of a cartridge memory conforming to an LTO standard prior to LTO 8 (hereinafter referred to as “conventional cartridge memory”) and is an area for storing information conforming to an LTO standard prior to LTO 8. The information conforming to an LTO standard prior to LTO 8 is, for example, manufacturing information (for example, a unique number of the cartridge 10A) or a usage history (for example, the number of times of tape withdrawal (thread count)).

The second storage area 336B corresponds to an extended storage area for a storage area of the conventional cartridge memory. The second storage area 336B is an area for storing additional information. Here, the additional information means information related to the cartridge 10A, not prescribed by an LTO standard prior to LTO 8. Examples of the additional information include tension adjustment information, management ledger data, Index information, and thumbnail information of a moving image stored in the magnetic tape 10, but are not limited to the data.

The tension adjustment information includes a distance between adjacent servo bands (a distance between servo patterns recorded in adjacent servo bands) at the time of data recording on the magnetic tape 10. The distance between the adjacent servo bands is an example of width-related information related to the width of the magnetic tape 10. Details of the distance between the servo bands will be described later. In the following description, information stored in the first storage area 336A may be referred to as “first information”, and information stored in the second storage area 336B may be referred to as “second information”.

The memory 336 may have a plurality of banks. In this case, some of the plurality of banks may constitute the first storage area 336A, and the remaining banks may constitute the second storage area 336B. Specifically, for example, in a case where the cartridge 10A conforms to the LTO-9 standard or the LTO-10 standard, the memory 336 may have two banks each having a storage capacity of about 16 KB. One of the two banks may constitute the first storage area 336A, and the other bank may constitute the second storage area 336B.

The antenna coil 331 induces an induced voltage by electromagnetic induction. The controller 335 communicates with the recording/reproducing device 30 according to a prescribed communication standard through the antenna coil 331. Specifically, for example, mutual authentication, transmission and reception of commands, and exchange of data are performed.

The controller 335 stores information received from the recording/reproducing device 30 through the antenna coil 331 in the memory 336. The controller 335 reads out information from the memory 336 in response to a request from the recording/reproducing device 30, and transmits the information to the recording/reproducing device 30 through the antenna coil 331.

(7) Effect

The magnetic recording medium 10 according to the present technology includes: the magnetic layer 13; the underlayer 12; the base layer 11; and the back layer 14, in which the number of recesses having a depth of 20% or more of the thickness of the magnetic layer 13 is 10 or more and 200 or less per surface area of 1600 μm², the base 11 includes a polyester as a main component, a surface on the back layer 14 side has a kurtosis of 2.0 or more and 4.4 or less, the magnetic recording medium 10 has an average thickness t_(T) of 5.6 μm or less, the magnetic recording medium 10 includes a lubricant, the magnetic recording medium 10 has pores, and the pores have an average diameter of 6 nm or more and 11 nm or less. As a result, an increase in the coefficient of dynamic friction of the magnetic layer side surface due to repeated recording or reproduction is suppressed, and winding deviation can be prevented. Suppression of an increase in the coefficient of dynamic friction of the magnetic layer side surface brings about maintenance of good traveling stability. For example, the magnetic recording medium 10 has good traveling stability even in a case where traveling has been performed at a high speed (for example, 8 m/s or more).

Moreover, the magnetic recording medium 10 also has excellent electromagnetic conversion characteristics.

(8) Modification

[Modification 1]

The magnetic recording medium 10 may further include a barrier layer 15 disposed on at least one surface of the base layer 11, as illustrated in FIG. 9. The barrier layer 15 is a layer for suppressing a dimensional change of the base layer 11 according to an environment. Examples of a cause of the dimensional change include the hygroscopic property of the base layer 11. However, by disposing the barrier layer 15, a penetration rate of moisture into the base layer 11 can be reduced. The barrier layer 15 includes a metal or a metal oxide. As the metal, for example, at least one of Al, Cu, Co, Mg, Si, Ti, V, Cr, Mn, Fe, Ni, Zn, Ga, Ge, Y, Zr, Mo, Ru, Pd, Ag, Ba, Pt, Au, or Ta can be used. As the metal oxide, for example, at least one of Al₂O₃, CuO, CoO, SiO₂, Cr₂O₃, TiO₂, Ta₂O₅, or ZrO₂ can be used, and any one of oxides of the metals described above can also be used. Furthermore, diamond-like carbon (DLC), diamond, and the like can also be used.

The average thickness of the barrier layer 15 is preferably 20 nm or more and 1000 nm or less, and more preferably 50 nm or more and 1000 nm or less. The average thickness of the barrier layer 15 is determined in a similar manner to the average thickness t_(m) of the magnetic layer 13. However, a magnification of a TEM image is appropriately adjusted according to the thickness of the barrier layer 15.

[Modification 2]

The magnetic recording medium 10 may be incorporated in a library device. That is, the present technology also provides a library device including at least one magnetic recording medium 10. The library device can adjust a tension applied to the magnetic recording medium 10 in a longitudinal direction thereof, and may include a plurality of the recording/reproducing devices 30 described above.

3. Examples

Hereinafter, the present technology will be described specifically with Examples, but the present technology is not limited only to these Examples.

In the following Examples and Comparative Examples, the squareness ratio S2 (without demagnetizing field correction) in the perpendicular direction, the squareness ratio S1 in the longitudinal direction, the pore volume and the pore average diameter (pore diameter of the maximum pore volume at the time of desorption), the total thickness, the average thickness t_(m) of the magnetic layer, the average thickness of the underlayer, the arithmetic average roughness R_(a) of the magnetic layer side surface, the number of recesses per 1600 μm² of the magnetic surface, the kurtosis Sku of the back surface, the friction coefficient ratio ((μ_(B)/μ_(A)), and the friction coefficient ratio (μ_(C(1000))/μ_(C(5))) are values determined by the measurement method described in “2. Embodiment of the present technology (example of application type magnetic recording medium)”.

(1) Manufacture of Magnetic Tape

As described below, magnetic tapes of Examples 1 to 19 and Comparative Examples 1 to 6 were manufactured. Table 1 below illustrates manufacturing conditions and measurement results.

Example 1

(Step of Preparing Magnetic Layer Forming Coating Material)

A magnetic layer forming coating material was prepared as follows. First, a first composition having the following formulation was kneaded with an extruder. Next, the kneaded first composition and a second composition having the following formulation were added to a stirring tank equipped with a disper, and were premixed. Subsequently, the mixture was further subjected to sand mill mixing, and was subjected to a filter treatment to prepare a magnetic layer forming coating material.

(First composition)

Powder of barium ferrite (BaFe₁₂O₁₉) particles (hexagonal plate shape, average aspect ratio 2.8, average particle volume 1950 nm³): 100 parts by mass

Cyclohexanone solution of vinyl chloride-based resin: 65 parts by mass

(The composition of the solution is 30% by mass of the resin and 70% by mass of cyclohexanone. Details of the vinyl chloride-based resin were as follows: (Degree of polymerization: 300, Mn=10000, OSO₃K=0.07 mmol/g and secondary OH=0.3 mmol/g were contained as polar groups.)

Aluminum oxide powder: 5 parts by mass

(α-A1203, average particle diameter: 0.2 μm)

(Second Composition)

Cyclohexanone solution of vinyl chloride-based resin: 1.1 parts by mass

(The composition of the solution is 30% by mass of the resin and 70% by mass of cyclohexanone.)

n-Butyl stearate: 2 parts by mass

Methyl ethyl ketone: 121.3 parts by mass

Toluene: 121.3 parts by mass

Cyclohexanone: 60.7 parts by mass

Carbon black: 2 parts by mass

(Manufactured by Tokai Carbon Co., Ltd., trade name: Seast TA)

Note that the n-butyl stearate was added as a lubricant.

Finally, 4 parts by mass of polyisocyanate (trade name: Coronate L, manufactured by Tosoh Corporation) as a curing agent and 2 parts by mass of stearic acid as a lubricant were added to the magnetic layer forming coating material prepared as described above.

(Step of Preparing Underlayer Forming Coating Material)

An underlayer forming coating material was prepared as follows. First, a third composition having the following formulation was kneaded with an extruder. Next, the kneaded third composition and a fourth composition having the following formulation were added to a stirring tank equipped with a disper, and were premixed. Subsequently, the mixture was further subjected to sand mill mixing, and was subjected to a filter treatment to prepare an underlayer forming coating material.

(Third Composition)

Acicular iron oxide powder: 100 parts by mass

(α-Fe2O3, average long axis length 0.15 um)

Vinyl chloride-based resin: 55.6 parts by mass

(Resin solution: resin content 30% by mass, cyclohexanone 70% by mass)

Carbon black: 10 parts by mass

(Average particle diameter 20 nm)

(Fourth Composition)

Polyurethane-based resin UR8200 (manufactured by Toyobo Co., Ltd.): 18.5 parts by mass

n-Butyl stearate: 2 parts by mass

Methyl ethyl ketone: 108.2 parts by mass

Toluene: 108.2 parts by mass

Cyclohexanone: 18.5 parts by mass

Note that the n-butyl stearate was added as a lubricant.

Finally, 4 parts by mass of polyisocyanate (trade name: Coronate L, manufactured by Tosoh Corporation) as a curing agent and 2 parts by mass of stearic acid as a lubricant were added to the underlayer forming coating material prepared as described above.

(Step of Preparing Back Layer Forming Coating Material)

A back layer forming coating material was prepared as follows. The following raw materials were mixed in a stirring tank equipped with a disper, and were subjected to filter treatment to prepare a back layer forming coating material. Carbon black powder having a small particle diameter (average particle diameter (D50) 20 nm): 90 parts by mass

Carbon black powder having a large particle diameter (average particle diameter (D50) 270 nm): 10 parts by mass

Polyester polyurethane: 100 parts by mass

(trade name: N-2304, manufactured by Tosoh Corporation)

Methyl ethyl ketone: 500 parts by mass

Toluene: 400 parts by mass

Cyclohexanone: 100 parts by mass

(Application Step)

Using the magnetic layer forming coating material and the underlayer forming coating material prepared as described above, an underlayer and a magnetic layer were formed on one main surface of a long polyethylene naphthalate film (hereinafter referred to as “PEN film”) having an average thickness of 4.12 μm as a nonmagnetic support such that the average thickness of the underlayer was 0.8 μm and the average thickness of the magnetic layer was 80 nm after drying and calendering as follows. First, the underlayer forming coating material was applied onto one main surface of the PEN film and dried to form an underlayer. Next, the magnetic layer forming coating material was applied on the underlayer and dried to form a magnetic layer. Note that the magnetic powder was subjected to magnetic field orientation in a thickness direction of the film by a solenoid coil when the magnetic layer forming coating material was dried. Furthermore, drying conditions (drying temperature and drying time) of the magnetic layer forming coating material were adjusted, and the squareness ratio S2 of the magnetic tape in the thickness direction (perpendicular direction) and the squareness ratio S1 thereof in the longitudinal direction were set to the values illustrated in Table 1. Subsequently, the back layer forming coating material was applied onto the other main surface of the PEN film and dried to form a back layer having an average thickness of 0.4 μm. As a result, a magnetic tape was obtained.

(Calendering Step and Transfer Step)

Subsequently, a calendering treatment was performed to smooth a surface of the magnetic layer. Next, the magnetic tape thus obtained was wound into a roll shape, and then the magnetic tape was heated at 60° C. for 10 hours in this state. Then, the magnetic tape was rewound in a roll shape such that an end located on an inner circumferential side was located on an outer circumferential side oppositely, and then the magnetic tape was heated again at 60° C. for 10 hours in this state. As a result, a large number of protrusions on the surface of the back layer were transferred onto the surface of the magnetic layer to form a large number of holes on the surface of the magnetic layer.

(Cutting Step)

The magnetic tape obtained as described above was cut into a width of ½ inches (12.65 mm). As a result, the target long magnetic tape (average thickness 5.4 μm) was obtained.

Example 2

A magnetic tape was manufactured by the same method as in Example 1 except that the drying conditions (drying temperature and drying time) of the magnetic layer forming coating material were adjusted, the squareness ratio S2 of the magnetic tape in the thickness direction (perpendicular direction) was set to 66%, and the squareness ratio S1 thereof in the longitudinal direction was set to 30%.

Example 3

A magnetic tape was manufactured by the same method as in Example 1 except that the drying conditions (drying temperature and drying time) of the magnetic layer forming coating material were adjusted, the squareness ratio S2 of the magnetic tape in the thickness direction (perpendicular direction) was set to 71%, and the squareness ratio S1 thereof in the longitudinal direction was set to 29%.

Example 4

A magnetic tape was manufactured by the same method as in Example 1 except that the drying conditions (drying temperature and drying time) of the magnetic layer forming coating material were adjusted, the squareness ratio S2 of the magnetic tape in the thickness direction (perpendicular direction) was set to 70%, and the squareness ratio S1 thereof in the longitudinal direction was set to 25%.

Example 5

A magnetic tape was manufactured by the same method as in Example 2 except that barium ferrite particles having an average aspect ratio of 2.3 and an average particle volume of 1300 nm³ were used instead of the powder of barium ferrite particles used in Example 1. The magnetic tape thus manufactured had an average pore diameter of 6 nm, and the number of recesses per 1600 μm² of the magnetic surface was 17.

Example 6

A magnetic tape was manufactured by the same method as in Example 2 except that the average thickness of the underlayer was changed to 0.6 μm and the heating temperature in the transfer step was changed to 70° C. The average thickness t_(T) of the magnetic tape was 5.2 μm. Furthermore, the number of recesses per 1600 μm² of the magnetic surface of the magnetic tape was 18.

Example 7

A magnetic tape was manufactured by the same method as in Example 2 except that barium ferrite particles having an average aspect ratio of 3.1 and an average particle volume of 2000 nm³ were used instead of the powder of barium ferrite particles used in Example 2.

Example 8

A magnetic tape was manufactured by the same method as in Example 2 except that the magnetic powder contained in the magnetic layer was changed from powder of barium ferrite particles to powder of ϵ iron oxide nanoparticles.

Example 9

A magnetic tape was manufactured by the same method as in Example 2 except that the magnetic powder contained in the magnetic layer was changed from powder of barium ferrite particles to powder of cobalt iron oxide nanoparticles.

Example 10

A magnetic tape was obtained by the same method as in Example 2 except that the average thickness of the underlayer was changed to 1 μm, the content of carbon black powder having a small particle diameter (average particle diameter (D50) 20 nm) contained in the back layer forming coating material was set to 80 parts by mass, and the content of carbon black powder having a large particle diameter (average particle diameter (D50) 270 nm) was set to 20 parts by mass. By changing the average thickness of the underlayer as described above, the pore volume of the magnetic tape was 0.023 cm³/g. Furthermore, the average thickness t_(T) of the magnetic tape was 5.6 μm.

Example 11

A magnetic tape was obtained by the same method as in Example 6 except that the pressure in the heat treatment after the calendering treatment was reduced, the average diameter of the pores of the magnetic layer was changed to 10 nm, and the average thickness of the underlayer was changed to 0.6 μm. The average thickness t_(T) of the magnetic tape was 5.2 μm.

Example 12

A magnetic tape was obtained by the same method as in Example 2 except that the pressure in the heat treatment after the calendering treatment was reduced, the content of carbon black powder having a small particle diameter (average particle diameter (D50) 20 nm) contained in the back layer forming coating material was set to 80 parts by mass, and the content of carbon black powder having a large particle diameter (average particle diameter (D50) 270 nm) was set to 20 parts by mass.

Example 13

A magnetic tape was manufactured by the same method as in Example 1 except that the drying conditions (drying temperature and drying time) of the magnetic layer forming coating material were adjusted, the squareness ratio S2 of the magnetic tape in the thickness direction (perpendicular direction) was set to 75%, and the squareness ratio S1 thereof in the longitudinal direction was set to 23%.

Example 14

A magnetic tape was manufactured by the same method as in Example 1 except that the drying conditions (drying temperature and drying time) of the magnetic layer forming coating material were adjusted, the squareness ratio S2 of the magnetic tape in the thickness direction (perpendicular direction) was set to 80%, and the squareness ratio S1 thereof in the longitudinal direction was set to 20%.

Example 15

A magnetic tape was obtained by the same method as in Example 2 except that the temperature in the calendering step was lowered.

Example 16

A magnetic tape was obtained by the same method as in Example 2 except that 60 parts by mass of carbon black powder having a small particle diameter (average particle diameter (D50) 50 nm) and 20 parts by mass of carbon black powder having a large particle diameter (average particle diameter (D50) 270 nm) were used as the carbon black powder contained in the back layer forming coating material.

Example 17

A magnetic tape was obtained by the same method as in Example 2 except that the content of carbon black powder having a small particle diameter (average particle diameter (D50) 20 nm) contained in the back layer forming coating material was set to 80 parts by mass, the content of carbon black powder having a large particle diameter (average particle diameter (D50) 270 nm) was set to 20 parts by mass, the heating temperature in the transfer step was changed to 70° C., and the pressure in the heat treatment after the calendering treatment was reduced.

Example 18

A magnetic tape was obtained by the same method as in Example 2 except that the content of carbon black powder having a small particle diameter (average particle diameter (D50) 20 nm) contained in the back layer forming coating material was set to 70 parts by mass, the content of carbon black powder having a large particle diameter (average particle diameter (D50) 270 nm) was set to 30 parts by mass, and the pressure in the heat treatment after the calendering treatment was reduced.

Example 19

A magnetic tape was manufactured by the same method as in Example 1 except that the drying conditions (drying temperature and drying time) of the magnetic layer forming coating material were adjusted, the squareness ratio S2 of the magnetic tape in the thickness direction (perpendicular direction) was set to 60%, and the squareness ratio S1 thereof in the longitudinal direction was set to 42%.

Comparative Example 1

A magnetic tape was obtained by the same method as in Example 2 except that the content of carbon black powder having a small particle diameter (average particle diameter (D50) 50 nm) contained in the back layer forming coating material was set to 100 parts by mass, the content of carbon black powder having a large particle diameter (average particle diameter (D50) 270 nm) was set to 0 parts by mass, the squareness ratio S1 in the longitudinal direction was set to 31%, and the temperature in the calendering step was raised. The pore volume of the magnetic tape was 0.015 cm³/g due to the above-described changes in the composition of the back layer forming coating material and conditions of the calendering treatment.

Comparative Example 2

A magnetic tape was obtained by the same method as in Example 1 except that the temperature in the calendering step was set to be higher than that in Comparative Example 1, and the pressure in the calendering step was set to be higher than that in Example 1.

Comparative Example 3

A magnetic tape was obtained by the same method as in Example 1 except that the temperature in the transfer step was changed to 50° C.

Comparative Example 4

A magnetic tape was obtained by the same method as in Example 2 except that 70 parts by mass of carbon black powder having a small particle diameter (average particle diameter (D50) 50 nm) and 30 parts by mass of carbon black powder having a large particle diameter (average particle diameter (D50) 270 nm) were used as the carbon black powder contained in the back layer forming coating material, and the pressure in the calendering step was reduced.

Comparative Example 5

A magnetic tape was obtained by the same method as in Example 2 except that 60 parts by mass of carbon black powder having a small particle diameter (average particle diameter (D50) 50 nm) and 40 parts by mass of carbon black powder having a large particle diameter (average particle diameter (D50) 270 nm) were used as the carbon black powder contained in the back layer forming coating material, and the pressure in the calendering step was reduced.

Comparative Example 6

A magnetic tape was obtained by the same method as in Example 2 except that the content of carbon black powder having a small particle diameter (average particle diameter (D50) 20 nm) contained in the back layer forming coating material was set to 70 parts by mass, the content of carbon black powder having a large particle diameter (average particle diameter (D50) 270 nm) was set to 30 parts by mass, the heating temperature in the transfer step was changed to 70° C., and the pressure in the pressure in the calendering step was reduced.

(2) Evaluation

For each of the magnetic tapes of Examples 1 to 12 and Comparative Examples 1 to 6 manufactured in (1) described above, the friction coefficient ratios (μ_(C(1000))/μ_(C(5))) and ((μ_(B)/μ_(A)) were measured. These friction coefficient ratios were measured by the measurement method described in “2. Embodiment (example of application type magnetic recording medium)”. The measurement results are illustrated in Table 1 below.

Moreover, the SNR was evaluated for each of the magnetic tapes of Examples 1 to 19 and Comparative Examples 1 to 6. The evaluation results are illustrated in Table 1 below. A method for evaluating SNR was as follows.

First, using a ½ inch tape traveling device (manufactured by Mountain Engineering II, MTS Transport) equipped with a recording/reproducing head and a recording/reproducing amplifier, the electromagnetic conversion characteristics (SNR) of each of the magnetic tapes were measured in an environment of 25° C. A ring head having a gap length of 0.2 μm was used as the recording head, and a GMR head having a shield-to-shield distance of 0.1 μm was used as the reproducing head. A relative speed, a recording clock frequency, and a recording track width were set to 6 m/s, 160 MHz, and 2.0 μm, respectively. Furthermore, the SNR was calculated on the basis of a method described in the following document. The results are illustrated in Table 1 as relative values based on the SNR of Example 12.

Y Okazaki: “An Error Rate Emulation System.”, IEEE Trans. Man., 31, pp. 3093-3095 (1995)

Moreover, the degree of occurrence of winding deviation was evaluated for each of the magnetic tapes of Examples 1 to 19 and Comparative Examples 1 to 6. The evaluation results are illustrated in Table 1 below. A method for evaluating the degree of occurrence of winding deviation is as follows.

A servo pattern according to the LTO7 standard is recorded on each of the magnetic tapes. Next, the magnetic tape is wound around a magnetic recording cartridge according to the LTO7 standard. The magnetic recording cartridge is caused to travel over a full length with a recording/reproducing device according to the LTO7 standard. Thereafter, a free drop test is performed on the magnetic recording cartridge according to JIS Z0200 that is a JIS standard (general rules of packaging cargo-evaluation test method). The drop height in the free drop test is set to 60 cm. In the free drop test, each of the drop orders 1 to 10 described in Table 11 of the JIS standard was performed once. For the magnetic recording cartridge after the free drop test, full-surface recording is performed by a drive according to LTO7.

In a case where winding deviation occurs by the free drop test, a portion where winding deviation occurs is broken and damaged in the full-surface recording. For this reason, a servo signal in the portion cannot be read, and recording is stopped, that is, the state becomes a fail state.

A magnetic tape that has become a fail state in the full-surface recording is determined as “defective”.

A magnetic tape that has completed full-surface recording over the full length without becoming a fail state in the full-surface recording is determined as “good”.

TABLE 1 Squareness ratio Pore Magnetic powder Squareness Squareness Average Aspect Particle ratio S2 in ratio S1 in Pore diameter Type of Shape of ratio of volume of perpendicular longitudinal volume of pores magnetic magnetic magnetic magnetic direction [%] direction [%] [cm³/g] [nm] powder powder powder powder [nm³] Example 1 65 35 0.02 8 BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 2 66 30 0.02 8 BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 3 71 29 0.02 8 BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 4 70 25 0.02 8 BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 5 66 30 0.02 6 BaFe₁₂O₁₉ Plate shape 2.3 1300 Example 6 66 30 0.02 8 BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 7 66 30 0.02 8 BaFe₁₂O₁₉ Plate shape 3.1 2000 Example 8 66 30 0.02 8 ε iron Spherical shape 1.1 2150 oxide Example 9 66 30 0.02 8 Co-iron Cubic shape 1.7 2200 oxide Example 10 66 30  0.023 8 BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 11 66 30 0.02 10  BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 12 66 30 0.02 8 BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 13 75 23 0.02 8 BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 14 80 20 0.02 8 BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 15 66 30 0.02 11  BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 16 66 30 0.02 8 BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 17 66 30 0.02 8 BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 18 66 30 0.02 8 BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 19 60 42 0.02 8 BaFe₁₂O₁₉ Plate shape 2.8 1950 Comparative 66 31  0.015 5 BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 1 Comparative 65 35 0.02 8 BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 2 Comparative 65 35 0.02 8 BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 3 Comparative 66 31 0.02 12  BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 4 Comparative 66 31 0.02 8 BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 5 Comparative 66 31 0.02 8 BaFe₁₂O₁₉ Plate shape 2.8 1950 Example 6 Number of Particle of back layer Thickness Roughness magnetic Small Small Large Total Thickness Thickness of R_(a) of surface Sku of particle particle particle thickness of layer underlayer magnetic 20% recesses/ back (D50: (D50: (D50: (μm) [nm] (μm) surface 1600 μm² surface 20 nm) 50 nm) 270 nm) Example 1 5.4 80 0.8 1.6 15 2.3 90 0 10 Example 2 5.4 80 0.8 1.6 15 2.3 90 0 10 Example 3 5.4 80 0.8 1.6 15 2.3 90 0 10 Example 4 5.4 80 0.8 1.6 15 2.3 90 0 10 Example 5 5.4 80 0.8 1.6 17 2.2 90 0 10 Example 6 5.2 80 0.6 1.7 18 2.3 90 0 10 Example 7 5.4 80 0.8 1.6 15 2.3 90 0 10 Example 8 5.4 80 0.8 1.6 15 2.3 90 0 10 Example 9 5.4 80 0.8 1.6 15 2.8 90 0 10 Example 10 5.6 80 1   1.6 15 2.3 80 0 20 Example 11 5.2 80 0.6 1.7 15 2.4 90 0 10 Example 12 5.4 80 0.8 1.7 17 3.3 80 0 20 Example 13 5.4 80 0.8 1.6 15 2.3 90 0 10 Example 14 5.4 80 0.8 1.6 15 2.3 90 0 10 Example 15 5.4 80 0.8 1.6 20 2.3 90 0 10 Example 16 5.4 80 0.8 1.7 22 4.2  0 60  40 Example 17 5.4 80 0.8 1.8 25 2.4 80 0 20 Example 18 5.4 80 0.8 1.7 160  3   70 0 30 Example 19 5.4 80 0.8 1.6 14 2.3 90 0 10 Comparative 5.4 80 0.8 1.5 14 2.2 100  0  0 Example 1 Comparative 5.4 80 0.8 1.5 15 1.8 90 0 10 Example 2 Comparative 5.4 80 0.8 1.5  9 2.3 90 0 10 Example 3 Comparative 5.4 80 0.8 1.6 20 2.6  0 70  30 Example 4 Comparative 5.4 80 0.8 1.7 50 4.5  0 60  40 Example 5 Comparative 5.4 80 0.8 1.8  210.0 3.8 70 0 30 Example 6 Friction Friction Transfer treatment coefficient coefficient Winding Temperature Time ratio ratio SNR deviation [° C.] [h] (μ_(B)/μ_(A)) (μ_(C(1,000))/μ_(C(5))) characteristics of tape        Example 1 60 20 1.2 1.2 0.1 Good Example 2 60 20 1.2 1.2 0.2 Good Example 3 60 20 1.2 1.3 0.4 Good Example 4 60 20 1.2 1.2 0.5 Good Example 5 60 20 1.2 1.4 1.2 Good Example 6 70 20 1.3 1.3 0.8 Good Example 7 60 20 1.2 1.2 0.1 Good Example 8 60 20 1.2 1.3 0.2 Good Example 9 60 20 1.2 1.2 0.3 Good Example 10 60 20 1.2 1.1 0.2 Good Example 11 60 20 1.2 1   0.2 Good Example 12 60 20 1.2 1.2 0.0 Good Example 13 60 20 1.4 1.5 0.6 Good Example 14 60 20 1.6 1.6 0.8 Good Example 15 60 20 1.5 1.4 0.2 Good Example 16 60 20 1.2 1.2 0.1 Good Example 17 70 20 1.2 1.2 0.4 Good Example 18 60 20 1.2 1.2 0.0 Good Example 19 60 20 1.2 1.3 −0.3   Good Comparative Example 1 60 20 2.2 2.3 0.0 Good Comparative Example 2 60 20 1.2 1.2 0.0 Poor Comparative Example 3 50 20 1.2 2.1 −0.5   Good Comparative Example 4 60 20 2.1 2.0 0.1 Good Comparative Example 5 60 20 1.2 1.2 0.1 Poor Comparative Example 6 70 20 1.2 1.2 −0.7   Good

The results illustrated in Table 1 indicate the following.

It is indicated that each of the magnetic tapes of Examples 1 to 19 has a low friction coefficient ratio (μ_(C(1000))/μ_(C(5))), and suppresses an increase in the coefficient of dynamic friction due to repeated recording or reproduction. Furthermore, each of these magnetic tapes had a good evaluation result regarding the degree of occurrence of winding deviation. These results indicate that the magnetic tape according to the present technology has a well-controlled frictional force with the head and is less likely to cause winding deviation although the magnetic tape has a thin total thickness.

Furthermore, comparison between the results of Examples 2, 5, 11, and 15 and the results of Comparative Examples 1 and 4 indicates that the friction coefficient ratio (μ_(C(1000))/μ_(C(5))) can be lowered by setting the average diameter of the pores to 6 nm or more and 11 nm or less.

Furthermore, the above comparison also indicates that the friction coefficient ratio ((μ_(B)/μ_(A)) decreases when the average diameter of the pores is 6 nm or more and 11 nm or less.

Furthermore, comparison between the results of Examples 2, 17, 18, and 19 and the results of Comparative Examples 3 and 6 indicates that setting the number of recesses having a depth of 20% of the thickness of the magnetic surface to, for example, 10 or more and 200 or less contributes to the low friction coefficient ratio (μ_(C(1000))/μ_(C(5))) and improvement in the SNR characteristics.

Furthermore, comparison between the results of Examples 1, 2, 12, and 16 and the results of Comparative Examples 2 and 5 indicates that the kurtosis of the back layer side surface within a range of, for example, 2.0 or more and 4.4 or less contributes to preventing occurrence of winding deviation.

Comparison between the results of Examples 1 to 4 and the result of Example 19 indicates that the SNR characteristics are further improved by setting the squareness ratio S2 to 65% or more.

The results of Examples 2, 6, and 10 indicate that the friction coefficient ratio (μ_(C(1000))/μ_(C(5))) is kept low and occurrence of winding deviation is prevented even in a case where the thickness of the underlayer is reduced to, for example, about 0.6 μm and in a case where the thickness of the underlayer is increased to about 1.0 μm, and further indicate that good SNR characteristics are obtained.

The results of Examples 2 and 7 to 9 indicate that an increase in the coefficient of dynamic friction is suppressed and occurrence of winding deviation is also prevented even if the aspect ratio and particle volume of the magnetic powder are changed or the type of the magnetic powder is changed, and further indicate that good SNR characteristics are obtained.

The results of Examples 1 to 4, 13, and 14 indicate that the squareness ratio S2 may be increased to, for example, about 75% and 80% in order to suppress an increase in the coefficient of dynamic friction and to prevent winding deviation.

Hereinabove, the embodiment and Examples of the present technology have been described specifically. However, the present technology is not limited to the above-described embodiment and Examples, but various modifications based on the technical idea of the present technology can be made.

For example, the configurations, the methods, the steps, the shapes, the materials, the numerical values, and the like exemplified in the above-described embodiment and Examples are only examples, and a configuration, a method, a step, a shape, a material, a numerical value, and the like different therefrom may be used as necessary. Furthermore, the chemical formulas of the compounds and the like are representative and are not limited to the described valences and the like as long as the compounds have common names of the same compound.

Furthermore, the configurations, the methods, the steps, the shapes, the materials, the numerical values, and the like in the above-described embodiment and Examples can be combined with one another as long as not departing from the gist of the present technology.

Furthermore, here, the numerical range indicated using “to” indicates a range including the numerical values described before and after “to” as the minimum value and the maximum value, respectively. Within the numerical range described step by step here, an upper limit value or a lower limit value of a numerical range in one stage may be replaced with an upper limit value or a lower limit value of a numerical range in another stage. The materials exemplified here can be used singly or in combination of two or more thereof unless otherwise specified.

Note that the present technology may have the following configurations.

[1] A tape-shaped magnetic recording medium including:

a magnetic layer; an underlayer; a base layer; and a back layer, in which

the number of recesses having a depth of 20% or more of the thickness of the magnetic layer is 10 or more and 200 or less per surface area of 1600 μm²,

the base layer includes a polyester as a main component,

a surface on a side of the back layer has a kurtosis of 2.0 or more and 4.4 or less,

the magnetic recording medium has an average thickness t_(T) of 5.6 μm or less,

the magnetic recording medium includes a lubricant, and

the magnetic recording medium has pores, and the pores have an average diameter of 6 nm or more and 11 nm or less when the diameters of the pores are measured in a state where the lubricant has been removed from the magnetic recording medium and the magnetic recording medium has been dried.

[2] The magnetic recording medium according to [1], having a squareness ratio of 65% or more in a perpendicular direction thereof. [3] The magnetic recording medium according to [1] or [2], in which a magnetic layer side surface of the magnetic recording medium has arithmetic average roughness Ra of 2.5 nm or less. [4] The magnetic recording medium according to any one of [1] to [3], in which the magnetic layer has an average thickness t_(m) of 80 nm or less. [5] The magnetic recording medium according to any one of [1] to [4], in which the magnetic layer includes magnetic powder, and the magnetic powder contains hexagonal ferrite, ϵ iron oxide, or Co-containing spinel ferrite. [6] The magnetic recording medium according to [5], in which the hexagonal ferrite contains at least one of Ba or Sr, and the iron oxide contains at least one of Al or Ga. [7] The magnetic recording medium according to any one of [1] to [6], having a friction coefficient ratio ((μ_(B)/μ_(A)) of 1.0 to 2.0, in which μ_(A) represents a coefficient of dynamic friction between a magnetic layer side surface of the magnetic recording medium and a magnetic head in a state where a tension of 0.4 N is applied to the magnetic recording medium in a longitudinal direction thereof, and μ_(B) represents a coefficient of dynamic friction between the magnetic layer side surface of the magnetic recording medium and the magnetic head in a state where a tension of 1.2 N is applied to the magnetic recording medium in the longitudinal direction. [8] The magnetic recording medium according to any one of [1] to [7], having a friction coefficient ratio (μ_(C(1000))/μ_(C(5))) of 1.0 or more and 1.8 or less, in which μ_(C(5)) represents a coefficient of dynamic friction at the fifth reciprocation in a case where the magnetic recording medium in a state where a tension of 0.6 N is applied to the magnetic recording medium in a longitudinal direction thereof is reciprocatedly slid five times on a magnetic head, and μ_(C(1000)) represents a coefficient of dynamic friction at the 1000th reciprocation in a case where the magnetic recording medium is reciprocated 1000 times on the magnetic head. [9] The magnetic recording medium according to any one of [1] to [8], in which the lubricant contains a fatty acid and/or a fatty acid ester. [10] The magnetic recording medium according to [9], in which the fatty acid contains a compound represented by the following general formula (1) or (2), and the fatty acid ester contains a compound represented by the following general formula (3) or (4):

CH₃(CH₂)_(k)COOH  (1)

where, in the general formula (1), k is an integer selected from a range of 14 or more and 22 or less;

CH₃(CH₂)_(n)CH═CH(CH₂)_(m)COOH  (2)

where, in the general formula (2), the sum of n and m is an integer selected from a range of 12 or more and 20 or less;

CH₃(CH₂)_(p)COO(CH₂)_(q)CH₃  (3)

where, in the general formula (3), p is an integer selected from a range of 14 or more and 22 or less, and q is an integer selected from a range of 2 or more and 5 or less; and

CH₃(CH₂)_(r)COO—(CH₂)_(s)CH(CH₃)₂  (4)

where, in the general formula (4), r is an integer selected from a range of 14 or more and 22 or less, and s is an integer selected from a range of 1 or more and 3 or less.

[11] The magnetic recording medium according to any one of [1] to [10], in which a coercive force in a longitudinal direction is 2000 Oe or less. [12] The magnetic recording medium according to any one of [1] to [11], in which the magnetic layer has five or more servo bands. [13] The magnetic recording medium according to [12], in which the ratio of the total area of the servo bands to the area of a magnetic layer side surface of the magnetic recording medium is 4.0% or less. [14] The magnetic recording medium according to [12] or [13], in which the servo band has a width of 95 μm or less. [15] The magnetic recording medium according to any one of [1] to [14], in which the magnetic layer can form a plurality of data tracks, and each of the data tracks has a width of 2.0 μm or less. [16] The magnetic recording medium according to any one of [1] to [15], in which the magnetic layer can record data such that a minimum value of a distance L between magnetization inversions is 48 nm or less. [17] The magnetic recording medium according to any one of [1] to [16], in which the base layer has an average thickness of 4.2 μm or less. [18] The magnetic recording medium according to any one of [1] to [17], in which the magnetic layer includes magnetic powder, and the magnetic powder has an average aspect ratio of 1 or more and 3.5 or less. [19] The magnetic recording medium according to any one of [1] to [18], in which the magnetic layer includes magnetic powder, and the magnetic powder has an average particle size of 50 nm or less. [20] The magnetic recording medium according to any one of [1] to [19], in which a surface on the back layer side has a kurtosis of 3.0 or more. [21] The magnetic recording medium according to any one of [1] to [20], in which the number of recesses having a depth of 20% or more of the thickness of the magnetic layer is 12 or more per surface area of 1600 μm². [22] The magnetic recording medium according to any one of [1] to [21], in which the number of recesses having a depth of 20% or more of the thickness of the magnetic layer is 14 or more per surface area of 1600 μm². [23] The magnetic recording medium according to any one of [1] to [22], in which the pores have an average diameter of 6 nm or more and 10 nm or less. [24] The magnetic recording medium according to any one of [1] to [23], in which the pores have an average diameter of 7 nm or more and 9 nm or less. [25] A tape cartridge including:

the tape-shaped magnetic recording medium according to any one of [1] to [24];

a communication unit that communicates with a recording/reproducing device;

a storage unit; and

a control unit that stores information received from the recording/reproducing device through the communication unit in the storage unit, reads the information from the storage unit according to a request from the recording/reproducing device, and transmits the information to the recording/reproducing device through the communication unit, in which

the information includes adjustment information for adjusting a tension applied to the magnetic recording medium in a longitudinal direction thereof.

REFERENCE SIGNS LIST

10 Magnetic recording medium 11 Base layer

12 Underlayer

13 Magnetic layer 14 Back layer 

1. A tape-shaped magnetic recording medium comprising: a magnetic layer; an underlayer; a base layer; and a back layer, wherein a number of recesses having a depth of 20% or more of a thickness of the magnetic layer is 10 or more and 200 or less per surface area of 1600 μm², the base layer includes a polyester as a main component, a surface on a side of the back layer has a kurtosis of 2.0 or more and 4.4 or less, the magnetic recording medium has an average thickness t_(T) of 5.6 μm or less, the magnetic recording medium includes a lubricant, and the magnetic recording medium has pores, and the pores have an average diameter of 6 nm or more and 11 nm or less when diameters of the pores are measured in a state where the lubricant has been removed from the magnetic recording medium and the magnetic recording medium has been dried.
 2. The magnetic recording medium according to claim 1, having a squareness ratio of 65% or more in a perpendicular direction thereof.
 3. The magnetic recording medium according to claim 1, wherein a magnetic layer side surface of the magnetic recording medium has arithmetic average roughness Ra of 2.5 nm or less.
 4. The magnetic recording medium according to claim 1, wherein the magnetic layer has an average thickness t_(m) of 80 nm or less.
 5. The magnetic recording medium according to claim 1, wherein the magnetic layer includes magnetic powder, and the magnetic powder contains hexagonal ferrite, 68 iron oxide, or Co-containing spinel ferrite.
 6. The magnetic recording medium according to claim 5, wherein the hexagonal ferrite contains at least one of Ba or Sr, and the 68 iron oxide contains at least one of Al or Ga.
 7. The magnetic recording medium according to claim 1, having a friction coefficient ratio (μ_(B)/μ_(A)) of 1.0 to 2.0, wherein μ_(A) represents a coefficient of dynamic friction between a magnetic layer side surface of the magnetic recording medium and a magnetic head in a state where a tension of 0.4 N is applied to the magnetic recording medium in a longitudinal direction thereof, and μ_(B) represents a coefficient of dynamic friction between the magnetic layer side surface of the magnetic recording medium and the magnetic head in a state where a tension of 1.2 N is applied to the magnetic recording medium in the longitudinal direction.
 8. The magnetic recording medium according to claim 1, having a friction coefficient ratio (μ_(C(1000))/μ_(C(5))) of 1.0 or more and 1.8 or less, wherein μ_(C(5)) represents a coefficient of dynamic friction at a fifth reciprocation in a case where the magnetic recording medium in a state where a tension of 0.6 N is applied to the magnetic recording medium in a longitudinal direction thereof is reciprocatedly slid five times on a magnetic head, and μ_(C(1000)) represents a coefficient of dynamic friction at a 1000th reciprocation in a case where the magnetic recording medium is reciprocated 1000 times on the magnetic head.
 9. The magnetic recording medium according to claim 1, wherein the lubricant contains a fatty acid and/or a fatty acid ester.
 10. The magnetic recording medium according to claim 9, wherein the fatty acid contains a compound represented by the following general formula (1) or (2), and the fatty acid ester contains a compound represented by the following general formula (3) or (4): CH₃(CH₂)_(k)COOH  (1) where, in the general formula (1), k is an integer selected from a range of 14 or more and 22 or less; CH₃(CH₂)_(n)CH═CH(CH₂)_(m)COOH  (2) where, in the general formula (2), a sum of n and m is an integer selected from a range of 12 or more and 20 or less; CH₃(CH₂)_(p)COO(CH₂)_(q)CH₃  (3) where, in the general formula (3), p is an integer selected from a range of 14 or more and 22 or less, and q is an integer selected from a range of 2 or more and 5 or less; and CH₃(CH₂)_(r)COO—(CH₂)_(s)CH(CH₃)₂  (4) where, in the general formula (4), r is an integer selected from a range of 14 or more and 22 or less, and s is an integer selected from a range of 1 or more and 3 or less.
 11. The magnetic recording medium according to claim 1, wherein a coercive force H_(c) in a longitudinal direction is 2000 Oe or less.
 12. The magnetic recording medium according to claim 1, wherein the magnetic layer has five or more servo bands.
 13. The magnetic recording medium according to claim 12, wherein a ratio of a total area of the servo bands to an area of a magnetic layer side surface of the magnetic recording medium is 4.0% or less.
 14. The magnetic recording medium according to claim 12, wherein the servo band has a width of 95 μm or less.
 15. The magnetic recording medium according to claim 1, wherein the magnetic layer can form a plurality of data tracks, and each of the data tracks has a width of 2.0 μm or less.
 16. The magnetic recording medium according to claim 1, wherein the magnetic layer can record data such that a minimum value of a distance L between magnetization inversions is 48 nm or less.
 17. The magnetic recording medium according to claim 1, wherein the base layer has an average thickness of 4.2 μm or less.
 18. The magnetic recording medium according to claim 1, wherein the magnetic layer includes magnetic powder, and the magnetic powder has an average aspect ratio of 1.0 or more and 3.5 or less.
 19. The magnetic recording medium according to claim 1, wherein the magnetic layer includes magnetic powder, and the magnetic powder has an average particle size of 50 nm or less.
 20. The magnetic recording medium according to claim 1, wherein a surface on the back layer side has a kurtosis of 3.0 or more.
 21. The magnetic recording medium according to claim 1, wherein the number of recesses having a depth of 20% or more of the thickness of the magnetic layer is 12 or more per surface area of 1600 μm².
 22. The magnetic recording medium according to claim 1, wherein the number of recesses having a depth of 20% or more of the thickness of the magnetic layer is 14 or more per surface area of 1600 μm².
 23. The magnetic recording medium according to claim 1, wherein the pores have an average diameter of 6 nm or more and 10 nm or less.
 24. The magnetic recording medium according to claim 1, wherein the pores have an average diameter of 7 nm or more and 9 nm or less.
 25. A tape cartridge comprising: the tape-shaped magnetic recording medium according to claim 1; a communication unit that communicates with a recording/reproducing device; a storage unit; and a control unit that stores information received from the recording/reproducing device through the communication unit in the storage unit, reads the information from the storage unit according to a request from the recording/reproducing device, and transmits the information to the recording/reproducing device through the communication unit, wherein the information includes adjustment information for adjusting a tension applied to the magnetic recording medium in a longitudinal direction thereof. 